Hate to ruin your party, but that information is fake.
These are famitsu numbers,
check it out
PlayStation 2 (56,878)
Game Boy Advance (30,029)
GameCube (8177)
Xbox (5184)
PS one (1663)
SwanCrystal (1123)
Wonder Swan Color (618)
Game Boy Color (575)
And these numbers are from IGN Japan
GameCube: 9,341
Xbox: 8,862
So there you have it, seems that other website you quoted took GC's famitsu numbers and Xboxes IGN Japan (which I belive is mediacreate) and did it that way.
Not that it matters, but you title is a bit misleading
.