Well there's no reason for you to have the same % for Malachite as Aventurine, unless for the Aventurine you had a buff in effect that wasn't in effect for the Malachite, as the recipes themselves are of a different "level", as are their base XP.
So getting 5XP for one and 1XP for the other at the same chance makes sense, so the only question is why you had 85% for each. I'd say either you mis-read on one or you had a buff in place, because they shouldn't give you the same chance, all things being equal.