Because it come pre-overclocked out of the Factory. Its a stupid PR thing, you could easily overclock the second card to the first card's speeds at home, and pay nothing for it!
true but the CO ships with a better cooler i think....however i don't know what the price difference is. don't even bother with the 7800 series anymore its going to become dead kind of like the 6800gt. the 7900gt is only a little more expensive