This is Puzzleing

Well I was looking at a GeForce FX5900XT today and saw something I thught was very strange. The clock speeds were "high." The 6800 OC card by BFG runs at 350/700 Mhz clock speeds. Now the FX5900XT has a much higher engine clock speed comming in at 450/700 Mhz. Now I understand there is alot more to these cards then just clock speeds that makes them great but why has Nvidia "downgraded" the clock speeds on their cards?

its the amount of pipes, the fx has either 4, I think, and the 6800 has OC, the clocks mean nothing compared to the extra four pipes of the 6800

Yeah I figured something like that was what was causing the better performance but why couldint they keep the same speeds as the older card or higher?