Mending is not coded properly.

When, for instance, wearing a full set of mending armor while repairing a mending item in your left or right hand, 4/5 of the xp will not go to the item you're trying to repair regardless of wether your armor is has full durability or not. Therefor one has to take of ones armor to repair the desired item as fast as possible. I find this to be a poorly design mechanic that could easily be fixed.