What others have mentioned, also like motherboards, some have higher quality Voltage components, more power phases etc. A extreme example is the higher quality, at least more expensive looking cooler and military grade components used on the Hawk 5770.
Though the powercolor's design probably has to meet , engineering specs set out in ATI's reference design, with some flexibility. In the past , by any given maker , there have been lemon models pushed out. Buy time decision !
Powercolor uses Arctic Cooler fans on that model and others, so its not like its 'cheap'.