If you re-evaluated your scores, vista runs the tests again. You might have had something running or taking up graphics power this time, or, it might be new drivers too. I updated my drivers one time, and dropped from a 4.5 to a 3.4. I saw no actual performance drop, in fact, it ran smoother after the driver update.

The numbers are just numbers. a good estimate that vista gives to the capabilities of your comp.