The Windows Experience Index (WEI) merely measures the relative capability of components. The WEI only runs for a short time and does not measure the interactions of components under a software load, but rather characteristics or your hardware.
Thus the WEI does not measure performance of a system, but merely the relative hardware capabilities when running Windows 7. An article about the WEI can be found here: http://blogs.msdn.com/b/e7/archive/2009/01/19/engineering-the-windows-7-windows-experience-index.aspx
In Vista, the WEI scores ranged from 1.0 to 5.9. In Windows 7, the range has been extended upward to 7.9.