Because MS was increasing the maximum score on the WEI scale, any existing scores had to be adjusted. Consider the following hypothetical scenario:
I have a video card which under Vista which currently scores 5.9. If I go and install Windows 7, where the maximum score is 7.9 the score of my existing hardware must be adjusted accordingly, since a lower end video card of relative performance should always score lower no matter what OS is being run, not higher simply because it was released later.
How would you feel if you had a Geforce GTX 295 which scored 5.9 in both Vista and 7, and a friend had a Geforce GTX - lets say for the sake of argument - 360 which scored 5.9 in Vista, but scored 7.1 in Windows 7?
Bottom line: If the scale increases, existing scores must be adjusted to match the new scale.
As other people have pointed out, existing scores will keep dropping relative to new hardware being released as well. But after having said all of this, i'm sure many of you reading this now know why I don't pay much attention to the WEI index.