its not purely a performance benchmark.
first if DX10 is not supported either by graphics card itself or drivers
(needs wddm 1.1) then the score is capped to 5.9
second the model of the graphics card can cap the score, I have a 9800gt and 8800gt tested in the same machine, they get the same raw benchmark result but one scores 7.3 and the other 6.9. Perhaps nvidia paid MS some money