Let me explain: the Samaritan Pentateuch (SP) is a version of the Torah that features several differences from the Jewish Torah. Among these differences are about forty major expansions to the Pentateuchal text, all consisting of material duplicated from elsewhere in the Pentateuch. For instance, in the Jewish Torah (Mosoretic Text--MT), God tells Moses to go warn Pharaoh that frogs are coming, and the next thing you read is that frogs are coming, but you never read that Moses went to warn Pharaoh. Well, in the SP, you do read that Moses warned Pharaoh; the text has been expanded with that conversation inserted. Similarly, when Moses reviews Israel's history in Deuteronomy 1–3, we encounter certain details that are not found in the MT version of Exodus or Numbers. The SP has those details inserted into Exodus and Numbers.
Aside from these major expansions, there is another category of differences between the SP and MT, consisting of Samaritan ideological alterations of the text--that is, someone committed to Samaritan theology, especially its emphasis on Mt. Gerizim as a holy site, inserted certain passages or minor revisions into the Torah to reflect this Samaritan theology. The number of definite Samaritan changes to the text is dwindling under scholarly scrutiny (see my recent article), but at least the Samaritan Tenth Commandment falls into this category.
Notice that the major expansions do not count as Samaritan ideological corrections. For one thing, they have nothing to do with particularly Samaritan theology. For another thing, almost all of these major expansions have been found among the Dead Sea Scrolls (definitely Jewish documents), especially 4QpaleoExod-m (4Q22), 4QNum-b (4Q27), 4Q(Reworked) Pentateuch (4Q158; 4Q364), and 4QEx-Lev (4Q17). Since these scrolls feature the major expansions preserved in the Samaritan Pentateuch, but not the Samaritan ideological insertions (i.e., the Samaritan Tenth Commandment), these Qumran scrolls (4QRP only secondarily) have been categorized as pre-Samaritan, under the assumption that it was one of these types of texts--current generally in Palestine during the last few centuries BCE--that the Samaritans chose as their base text before inserting some of their own ideological alterations into the text.
Now, it seems clear that the Samaritans chose their text deliberately, because most of the books in their Pentateuch bear the same character--expansive texts. (Not the most expansive versions of the respective books, mind you: 4Q158, 4Q364, and other scrolls feature even more additional material.) Sometimes you'll read that the Samaritans chose an expansive text "in all five Torah books" (Tov, Textual Criticism, 93), but this is not technically accurate, because there's no such thing as an expansive text of Leviticus. Eugene Ulrich, who has emphasized more than anyone the pluriform character of ancient scriptural texts and has pushed scholars to think in terms of variant literary editions for ancient scriptural books, says that there are no variant literary editions attested for Leviticus, just the one long known from the MT (see his essay in Tov's Festschrift, p. 459).
But then I find that Tov thinks maybe there is a Leviticus scroll in the expansive tradition. So, why does he think 4QLev-d belongs to this tradition? The main reason seems to be there is a substantial expansion found in the text at Lev 17:4. Here's the text in the Dead Sea Scrolls Bible (p. 95), with the additional material (vis-a-vis MT) in italics.
and [has not brought it] to the door of the tent of meeti[ng so as to sacrifice it as a burnt offering] or an offering of well-being to the Lord to be acceptable as [a pleasing odor, and has slaughtered it without and] does not bring it [to the door of the tent of me]eting to offer it as an offering to the Lord before the taber[nacle] of the Lord...The SP and the LXX also attest this expansion. But that's about it for the scroll. There are a few other minor variants. The whole scroll is not very big and contains only parts of Lev 14:27–29, 33–36; 15:20–24; 17:2–11.
It seems unlikely that 4QLev-d does belong to the expansive group, or certainly that it should be classified as pre-Samaritan, without further evidence. It doesn't really conform to the pattern: yes, it has a major expansion shared by SP, but that same expansion is also shared by LXX. But the LXX doesn't share the SP expansions as a rule. Though the LXX and SP frequently agree against MT in minor variants, they do so in none of the other major expansions. In fact, elsewhere Tov has classified the scroll as in the LXX group (see here, p. 297). Neither does it conform to the pattern in the sense of duplicating material found elsewhere in the Pentateuch. Magnar Kartveit doesn't even count this case as one of the major expansions in SP, presumably because of its deviation from the pattern. Eugene Ulrich considers this reading to be an "isolated supplement," not indicative of a variant literary edition.
This case may be compared to that of 4QDeut-n. Scholarly consensus says that 4QDeut-n is not a pre-Samaritan text (Elizabeth Own, Sidnie White Crawford), a view with which Tov (91n147) is in agreement. This view makes sense because 4QDeut-n doesn't actually share any major expansions with SP. But it does share the method of expansion. That is, 4QDeut-n has only one major expansion: in the Ten Commandments (Deut 5), our scroll combines the Sabbath command justification from Exodus (creation) with the justification from Deuteronomy (Egyptian slavery), so that both justifications appear in the Deuteronomy text. This expansion is not known in any other text, including the SP. But the method of expanding the text is very similar to the way the SP expansions operate. Indeed, the method here is much more similar to the major expansions of SP than is the method of the expansion of Lev 17:4 in 4QLev-d. Based on this example, one might want to label 4QDeut-n as pre-Samaritan. After all, 4QNum-b contains an expansion (in ch. 36) not attested in the SP, but it still gets the label pre-Samaritan because all of the SP expansions in Numbers are found in 4QNum-b. Who's to say whether--if we had more of 4QDeut-n--it would not also contain the expansions of SP Deuteronomy? There are only two expansions in SP Deuteronomy at Deut 2:7 and 10:6, both duplicating material from Numbers. 4QDeut-n is not extant for these passages. The scroll probably never contained the entire text of Deuteronomy, but was instead an excerpted text. The only preserved portions of text are Deut 8:5–10 (col. 1) and Deut 5:1–6:1 (cols. 2–6). At any rate, it could have received the label pre-Samaritan on the basis of method of expansion, under the assumption that had more of the text been preserved, there probably would have been overlaps with the expansions in SP. And the 4QDeut-n expansion not attested in SP would be like the 4QNum-b expansion not attested in SP. Nevertheless, on the basis of lack of evidence, 4QDeut-n gets the label non-aligned because it doesn't line up with the MT or the SP or the LXX.
The case with 4QLev-d is different, but not completely dissimilar. We don't have much of the text. What we do have features a shared expansion with SP and LXX, but there is no overall pattern to expansion in Leviticus, so that even Ulrich wont go so far as to say there are variant literary editions for this book.
All that to say, I consider it very dubious that 4QLev-d belongs to the pre-Samaritan group, despite Tov's opinion that it possibly does so.