Splash Potion Of Instant Healing

I don't know if this is a bug or a ingame minecraft thing, but while I was playing, I was bored, and wanted to throw Splash Potions at mobs, and I noticed that most of the mobs don't get affected by the potion. I tested all of the mobs, and I only noticed Skeleton, Zombie Pigman, and Zombies get affected. As again, I think this is a bug, but I'm not really sure.