Anvil Cost Not Reseting

I was trying to Max out my Armor when I realized that I couldn't add Thorns on my helmet if I had Aqua Affinity on it. When i got all the enchantments i needed, i used the helmet on a grindstone and went to put the enchantments on. But the Anvil showed me that it didn't reset the Enchant cost. Is this a Bug or can you never reset the cost of an item?