My 4870 (with 1GB) gets 6.8 for both graphics scores.
I suspect that there is something else being measured than pure GPU performance. For example, if you have onboard graphics, it may calculate Aero based on that since it would require less power to run from there.
I got that score higher
Gaming graphic = 6.4
windows aero, transparency on = 4.2
windows 7 basic = 4.4
Windows clasic = 5.6
so just by changing the apearance it gives a boost of 1.4 visuall effects didn't change anything.