Anonymous:

When used in conjunction with EnderZoo the rebirths health gets multiplied. This has to do with the difficulty level modifiers built into EnderZoo.

If enderzoo is doing something to the mob's health after it spawns, there isn't much I can do to stop it. After a mob is reborn it gets spawned back into the world, which is when I'd guess Ender Zoo is modifying the health. Since the health is already modified, it gets multiplied.