Throwing Bottles o' Enchanting into a one block high ceiling causes XP to end up at the top

I noticed xp orbs appearing outside walls where they shouldn't be, so I built an enclosed glass box around me 5 x 5 with the roof 4 blocks above. Then I grabbed 3 or 4 Bottle o' enchanting and hurled them towards the ceiling. Most of the orbs ended up outside the box on the roof.

I tried the same with an enclosed cobblestone box and the same happened.