The Windows Experience Index is a tool found on the system properties window that determines the performance of your computer's hardware by running tests on them. The values are set on a scale with a maximum of 7.9 on Windows 7. The base score is determined by the system's bottleneck score.
Since Windows 8, the scale ranges up to 9.9, changing your old score on the same system. This is why your computer may score worse with a Windows 8 system than with your old Windows 7 system. My score went from 4.6 to 4.2 from Windows 7 Enterprise to Windows 8 CP with the same hardware installed: