Why is the term pureblood used for the lowest rank in the yuan-ti caste system?

In Volo’s Guide to Monsters, the chapter on Yuan-ti tells us that they tend to live in a caste system, where the Abominations are the top rank, the Malisons† are the middle rank, and the purebloods are the lowest rank. From p. 92:

The vast majority of yuan-ti fall into three categories–abominations, malisons, and purebloods–while the mutated broodguards and exceedingly rare anathemas have their place in the hierarchy as well.

This is further expanded on p. 95, but there is something that is missing from their explanation: Why is the term "pureblood" used to describe the lowest rank?

Intuitively, the term "pureblood" makes me think of the highest rank, as villainous caste-based societies often consider their highest members "pure" and those beneath them somehow "impure" (I’m probably thinking of vampires, but I can’t remember from which fiction).

However, when it comes to Yuan-ti, the term "pureblood" is used for their lowest rank. So does that mean that their higher ranks are "impure"? Why are they considered better then, if they are "impure"? I mean, I know that the fact that the higher ranks are more snake-like is why they are considered "better" by the Yuan-ti, but to me the terminology does not match up.

The information in the Monster Manual and Volo’s Guide to Monsters describes the purebloods as being the most "human-like", but they are still Yuan-ti, not actually human, so that seems to exclude the fact that they are "pureblooded humans" or something; if anything, it implies their blood is weaker, since they’re only a little part snake and mostly human (but not entirely human). Surely then, those who have not gone through any transformation at all should be called "purebloods"? Anyway, these ramblings are not my question, they are just to show my train of thought and confusion.

Is there anything published in official material, presumably in the lore from earlier editions of D&D (unless I’ve overlooked something in 5e), that explains why the term "pureblood" is appropriate for their lowest rank, or that otherwise explains why that makes sense? I’m looking for something that explains the in-universe lore reason behing this name, not the real-life reason as to why that name was chosen for that monster. Lore answers only please.

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† Whilst playing the video game Neverwinter Nights 2: Storm of Zehir, which is based on D&D 3.5e, a loading screen message tells me a brief version of the same information:

LORE: There are three major castes within yuan-ti society: the purebloods, halfbloods, and abominations. The more snakelike the individual, the higher their status.

Therefore, I assume Malisons in 5e were called Halfboods in 3.5e, or that they are somehow otherwise equivalent.