There is a bug where a mob will randomly spawn with a lot more HP than it should have. For example, a level 2 zombie with 75 HP, which is way more than others that have normal health. It can be a mob of any level and the HP value can also vary but its always just a lot more than it should be. I'm not sure if these mobs are gaining some other attribute bonuses above what they're supposed to be but HP is just the easiest to notice.