Bugged Anvil Enchanting (Anvils are not as cheap as i hoped lol)

When you make an enchanted sword with say Sharpness 10 (illegal enchant)
and then try 2 enchant it in an Anvil the Sharpness goes down 2 Sharp 5.
This is not cheap or cheating because u have 2 cheat just 2 get
the enchanted sword. This bug thing affects all versions but the create issue would not let me say 1.6.4. omg another bug. Yay