Erratic Level Cost When Combining Enchantments

Cost for combining enchanted items using the Anvil is erratic.

Steps to Reproduce:
1. Player makes two Pickaxes of the same item tier,
2. Player gives the first Pickaxe an Unbreaking 1 enchantment,
3. Player gives the second Pickaxe an Efficiency 1 enchantment,
4. Player activates the anvil,
5. Player places the Efficiency 1 Pickaxe in the first slot, and the Unbreaking 1 Pickaxe in the second slot, and notes the Enchantment Cost (8),
6. Player removes both Pickaxes and puts them in in the opposite slots,
7. Player observes that the level cost has changed (should be 7 levels).

Expected Results:
Combining enchantment's cost should not be different depending on what slot each item is in.