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Back when Xbox 360 was released, It was the only next gen console of the time and consumers aren't as ready in upgrading consoles than when PS3 was released. This coupled with the fact that everyone was was waiting for the much hyped PS3, Xbox 360 was struggling with consumer acceptance.
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==> it is obvious that Gears of War, particuylarly in NA was sufficient to sell the console very well whatever the PS3 was released or not.
If the PS3 is behind in this year to year comparaison it is mainly because of the NA numbers that clearly show that NA consumers bought the Xbox360 without thinking to the PS3.
EDIT : just look the NA numbers for black friday, year to year, you will see it is quite close ...
Time to Work !