Level 30 enchants only taking away 3 levels of XP

I had gotten level 30, and went to go enchant. This was my first time enchanting in 1.8. I put my bow in, 3 lapis as was required, and selected level 30 enchant. After I had finished I noticed I still had level 27. I went to my mob grinder, got 3 more levels, came back, and enchanted something else at level 30. Same thing. After several more time, it appears that level 30 enchants only take away 3 levels of XP. I also created a new world and tested it, same thing, so I thought I would report it. It's really over-powered only needing to get 3 levels for a max enchant.