Okay, so I was playing in my survival world (14w11b) and decided to go wither skeleton skull hunting. I started killing some of them with my Looting III sword and from 4 skeletons I killed, 3 of them dropped a skull; furthermore, I've got myself a considerable ammount of them so far. That's why I was wondering if this is a bug, the chances of this drop have been changed or just the upgrade of Looting enchantment is what's causing it.