It's a myth that Xbox did any better than GameCube (sales) as is clearly evidenced by this chart:
http://vgchartz.com/hwcomps.php?cons1=GC®1=All&cons2=XB®2=All&cons3=Wii®3=All
Microsoft spin will tell you that XB was a success but it didn't do any better than GC and ended up selling slightly fewer consoles.
Just goes to show that gamers will believe what they are told to.