Microsoft first created the Windows Experience Index (WEI) for Windows Vista, designing it as a system in which the performance of several key hardware components are rated and an overall score is provided. (That overall score matches that of the lowest-scoring component, the theory being that the weakest link in the chain represents your PC's "true" performance score.) Updated first in Windows 7, WEI has now been updated for a second time in Windows 8.
And it's somewhat amusing. In Windows 8, the WEI score (and the various subscores) now goes to 9.9.
Here's a shot of the interface, which hasn't really changed.
WEI from Windows 8 Consumer Preview (Samsung Series 7 tablet PC)
You may recall that the original WEI system, from Windows Vista, rated the system processor, memory, graphics, gaming graphics, and primary hard disk on a scale from 1 to 5.9. In Windows 7, this scale was raised on the high-end to 7.9. And now in Windows 8, it's 9.9.
The changes are made in each version to account for improvements in hardware and so that the scores, over time, are consistent.
Still, 9.9. Who comes up with this stuff?