When enchanting a enchant that cost more than 3 XP bug.

Every time I enchant an item from an enchanting table, that costes more than 3 levels it doesn't take the right about of XP doesn't matter how much lapis cost.

EX. An enchant that cost 6 only takes away 5 XP
Ex. An enchant that cost 4 only takes away 3 XP ,etc.