Some humans have functional extra digits. (Others have partial digits that are more vestigial.) The original number of polydactyl births is about 1 in 500, but you almost never see any due to erasure, because the vast majority are surgically modified shortly after birth to resemble everyone else. Those who manage to keep their extra digits, however, can often do things that other people can't.
They have the Extra Body Part(s) superpower. It's just not acknowledged that way. It still lets them do more.