probably because your gtx is not good enough...just kidding!
i wouldn't even pay attention to the vista rating thing, it doesn't really know what it's talking about in many regards. As long as actual in game performance isn't being compromised, then i wouldn't worry about it.
Well, for the graphics assessments.... yeah, it doesn't know what it is talking about a lot of times. My Radeon X300 graphics cards was rated as a 2.0 for the business assessment and a 3.0 for the gaming assessment..... which is pretty darn freaked up, because the X300 should do business stuff and Aero better than gaming stuff!