Well, a combination of giving him the higher bounty back in Sabaody and that he has Conqueror's Haki, but Law doesn't, so that gives him a little boost in portrayal. Also, Kidd's fruit should be absolutely broken as well, but he uses it like shit, Law has some endurance problems, i think Law > Kidd, but it doesn't seem like that was Oda's objective back in Sabaody.