The anvil is kinda bugged, when you put some item into it, it sucks more exp than is showing on the screen.

Example:Reparing a Diamond Sword with some diamonds. Required:10 levels. And then you get the item all repared, but you just realized that the anvil didn't suck 10 levels, it sucked almost 20!

My friend said that, very rare, the anvil sucks your exp sometimes without even reparing your item.