While trying to get animals into a fenced area so they wouldn't wander around they would lose interest in the food being held if their is any change in elevation; jumping a block up, falling a block down, walking up/down a slab block or stairs block, walking over dirt/grass onto a fence block...

They lose interest mid-jump/mid-fall making it harder to move them from one area to another, unless they are on the same level of ground.