anvil repair bug, order of items changes req. XP although same result

in Anvil, put the following:

slot 1: damaged "Bow Unbreaking 1"

slot 2: very damaged simple "Bow"

slot 3 result: "Bow Unbreaking 1"

required XP: 3

 

now change order:

slot 1: very damaged simple "Bow"

slot 2: damaged "Bow Unbreaking 1"

slot 3 result: "Bow Unbreaking 1" (exact same repair status as in 1st order)

required XP: 5 ...