I was creating an enchanting space with 25 bookshelves and an enchanting table, I checked to see what max enchant level I had for 25 bookshelves, and it was 30. I then decided to decorate around my space by placing carpet, and then I tried to enchant a diamond pickaxe. I could only get 1-8 levels after I placed the carpet down, when I should have been able to get 30. I had 32 experience levels for enchanting.
I placed carpet between the table and the bookshelves and then the enchant level fell considerably. The table was set up with 8 blocks around it and 22 bookshelves around it. the bookshelves were 2 tall and there were 3 bookshelves at 3 block height. I assume that those 3 at block height 3 still contributed to the enchant table.