I can think of a couple of reasons ~I think I've missed something, but how come 480 didnt have aib cards since day 1 like 1060? (and still doesn't). Almost same situation as when r290 reference throttlers were compared to nvidia OC cards.
The stock 480 were AIB/OEM/Apple rejects therefore the card (ASIC) quality was generally lower & hence they required overvolting, at least in some cases, to be sold as 480 & not 470.
The AIB's wanted/needed better quality chips to compete against the 1060 & reference 480, given better efficiency & more OC room they'd sell at a premium & rightfully so, not like the FE crap.