Well, with ATI partnering with other companies, it is possible that the other, non-ATI 8500s and the like may not be as sharp as ATI's own 8500. It's partially due to some engineering, not the chip.

Same reason some companies can manage to make an Intel chipset based board suck hard or make a Via board shine. It's partially the chip, and partially the company that makes the product with the chip in it.