Either I am confused or this is bogus, theaveng.
You wanted to find the average score, but the site you are using for your information does not list even half the games for any of the 3 systems. Only 30 Xbox, 33 GameCube, and 38 PS2 games reviewed.
For instance they don't have
Star Wars Rogue Squadron II: Rogue Leader listed. (IGN gave this game a 9.1)
Or how about
Jak and Daxter: The Precursor Legacy? (IGN gave this one a 9.4)
I'm not a big fan of IGN. I'm just using them as an example because it is one of Gametab's sources.
Looking at your posts above, you are using the 30 or so games you calculated an average for to represent the entire library for each system.
You're wrong.