Yeah, which makes it difficult to judge who's the greatest necromancer according to their biggest ritual accomplishments, so we kinda have to look at other achievements and character vibes, like how Kemmler's shtick is having built upon all the knowledge of his forebears to the point of creating his own Lore of Necromancy+10.
I think it's a bit silly to say that because a wizard was able to innovate on their lore it means that they must have been more powerful then their predecessors, but even then, even looking back at the lore of the Lichemaster, army of the Lichemaster describes it as such:
Being a Necromancer of almost incomparable power, Kemmler utilises more powerful versions of the spells of Undeath.
Meaning that even Kemmler's own background description feels the need to tell us that as powerful a necromancer as Kemmler is, he cannot be said to have power that's incomparable to all other necromancers, which is saying something given how fond Warhammer writing is of using purple prose to hype up the character it is describing at the time.
What necromancer could have been so powerful as to deny Kemmler the title of supreme necromancer? Perhaps the one whose spell books remain as coveted artifacts by even some of the most powerful necromancers in the setting in their own right such as Vlad Von Carstein, Melkhior and Zacharias the Ever Living? Who forged some of the most powerful magic items the setting has seen? Who committed the largest casting of necromancy in the setting and whom even the proud high elves dare not say that even the greatest of their living wizards can surpass him?
No, Morathi took part in Malekith's massed covens of warlocks, who collectively caused the Sundering.
That kind of goes back to the point that most of the castings of great rituals seem to have been collaborative acts. At the top of my head the only ones of the greatest rituals in Warhammer who were cast singlehandedly are Nagash's great ritual of reawakening and the Deliverance of Itza by lord Kroak, and if we discount Nagash's ritual due to all the warpstone he had available then that just leaves lord Kroak, though I suppose saying that Kroak was at level of his own in terms of magical power is a reasonable point, but then it doesn't do much when comparing all the wizards competing for the "rankings" after him.
The point is that when each book repeats the claim right down to the same phrasing, then at some point you have to acknowledge that this isn't entirely a considered and deliberate lore statement, and may well have been at least in part just copying and pasting old material to save time.
How is a lore point being repeated through multiple army books mean it is not considered and deliberate? Unlike something that appears in a single army book, it means that said statement is considered so solid that it was held to be true even through multiple editions. Compare that to the far more brief existence of the lore of the Lichemaster, which was removed entirely in later editions.
