Anvil not comibing enchants correctly

I have two iron helmets. One with Protection III, and one with Unbreaking II and Blast Protection II. If I try to combine them on an Anvil, depending on the order, my options are either Protection II and Unbreaking II, for 8 levels, or the same enchants for only 2 levels. It will not let me add blast Protection II to the Protection III and Unbreaking II.