I doubt you'll find a direct match between those cards. The 7600GT was nearly as fast as the 6800GT. You might be able to find a comparison between the 7600GT and the 8500GT. My guess would be overall they will be close. (8500GT and the 6800GT.)
As for why some cards count or don't, I wouldn't worry about it. My guess is that seeing as people can't list EVERY card they have to just list something. They might list a card because it has something you need, like a card with SM3. No x8xx cards, but an x16xx would work. Even if the x850XT is faster, it lacks SM3.