360 platform.
Assassin's Creed has an overall of 81 on metacritic, it sold 4.53 million.
Fable 2 overall 89, sold 3.21 mil.
Mass Effect overall of 91, sold 2.08 mil.
Fallout 3 overall 93, sold 2.63 mil.
Oblivion overall 94, sold 2.84 mil.
It seem pretty random to me, I don't think metacritic score has that big of a say in sales numbers.
Do you have a source for these sales numbers?