Yes, I have the answer. I noticed a while back that several manufacturers were selling the GTS with the PRO chip and calling it the PRO. To be a true PRO, it needs to have 5.5ns DDR SDRAM. But on a lot of websites you'll notice that they actually tell you that it has 6ns memory, or else they don't tell you at all. The PCB revision makes sense as the achitecture is the same, meaning that a manufacturer could even make a true pro on a GTS PCB by simply using the PRO chip and 5.5ns memory. But they don't. You may notice that on many sites that say 5.5ns, they also say "specifications subject to change without notice". It's a rip off. I first became suspicious when I saw Pro cards selling for the same price as GTS from several manufacturers. So you got taken, and because of the "specifications subject to change without notice" clause, there is nothing you can do about it but cry.
If you pulled the heatsink, I bet you would find that the chip itself is a PRO chip, but if you look at the memory, it's 6ns, which is made to run at 333MHz DDR instead of 400. I warned people of this several moths ago, a few said I didn't know what I was talking about and the rest didn't even bother to respond.
Cast not thine pearls before the swine