Is the Windows Experience Index score calculated the same way on Windows 7 and 8?
No, it isn't.
The highest possible value in Windows 7 is 7.9
.
The highest possible value in Windows 8 is 9.9
.
Are the scores given on Windows 7 and 8 the same all the way up to 7.9, or are they calculated differently?
They're calculated differently.
Here is a report from somebody whose WEI dropped from 4.6
to 4.2
.