I think that Nvidia (however the capitalization is) and ATi have a speed for each chipset that they would consider "stock," a speed at which they guarantee the chip will run stably. Those are the speeds at which most cards using that chip will run. However, factory overclocked cards have their chips running faster than the manufacturer recommends. Just like if you buy a PC with a Sempron 3000+ socket 462 (2 GHz recommended by AMD) but the PC manufacturer would have it OCed to 2.5GHz.
