Anvil Difference in Cost

I am not sure if this is a bug or something that I will just have to get over, but when using an anvil, if I add my enchanted axe to the first slot, and a non enchanted axe of the same material to the second slot, the cost is relatively small, around 2-4. However, when I reverse these two items, the enchantment cost sky rockets to 18-24 enchantment cost.