Windows experience index means jack **** for example it says that my HD5850 clocked at 1Ghz core and 1200mem is 6.9 for gaming where as your getting 7.4 with a 4870 and obviously a 5850 is faster than a 4870.
What matters is 3dmark05
But seriously what matters is game performance, don't worry about windows experience index.
Probably an evil scheme between ATi and MS to make you ditch your 4870 and get a new 5870. But seriously, the drivers must work worse for however Windows tests your GPU. I got a 7.2 rating with a HD 4770 @ stock on 10.1 drivers, and honestly I really wouldn't care what Windows says. I'd care what FRAPS says about Crysis.