"In the new MM errata, the demilich has the following change:
> Demilich (p. 48). Hit points: 80 (32d4) [was 20d4].
This change makes sense normally. 32*2.5=80, whereas 20*2.5=50.
The issue, though, is that the demilich has a block of flavor text in its description, that says roughly the following (I'll copy the full text when I consult my MM):
> "The demilich has such a strong will to live that it takes the maximum roll when it rolls for max HP"
This means that a demilich is supposed to take 20*4=80 when rolling for HP.