Drain part

So, I was draining the black dragon right, and I saw it only healed me half way of my ranks healing. I'm rank 5 and I only healed up, like, 165. Are some monsters resistant to the drain effect? Or does it do full damage, but only heals for half? It's really hard for me to tell since all the ones that I found that do this have millions of hp, and it's sort of hard to tell. Monsters I've seen do this so far is black dragon and red dragon. I have to drain the raptors get to a full heal out.

Well, I got some more data. Protection and stuff definitely isn't a factor in lowering the amount healed, but protection definitely seems to be affecting the knock back damage. On white poisonous snake my knock back damage did ~640 damage, but on amethyst golems it did ~240 and the same with sea trolls.