anvils dont do enough damage

Can confirm. It would seem that the maximum damage done to zombies (and likely other mobs) is 9 hearts, instead of 10. You will notice that doing the same on a player wearing no helmet will kill them instantly. I cannot find anything that suggests this is intended, as this contradicts what is written on the minecraft wiki.

The anvils, testing them on 13w05a, will kill zombies and other mobs, from a height of 15 on easy.

Same results, resolving.