Anvils not enchanting properly

If i have a protection 3 piece of diamond leggings , and i try to enchant them with a protection 1 book in an anvil , it eats up my xp but doesn't enchant them at all , and if you try to do /enchant @p protection 1 it says "diamond leggings cannot support this enchantment" , but it works if you have no enchants on them and then type /enchant @p protection 1