1. A year is 52 weeks.
2. The gap between weekly sales never stays the same, it is perfectly possiblefor the PS3 to pass the X360, lets say 2009 the 360 outsells the PS3 for the first 9 months, racks up a largest gap of about 10 million, then a PS3 price cut brings new lustre to PS3 sales, it catches up in weekly sales, then manages to outsell it during the holidays by say 1 million.
end of 2009 gap = 9 million
By 2010 the X360 is quite old in the eyes of consumers, it starts to slow down quite a bit, sells maybe 7 million even with a further price cut. The PS3, having delayed it's 3rd year on the market being the best due to no price cut, manages to have a 4th year equal or slightly higher than it's second year, so catches up by about 4 million
end of 2010 gap = 5 million
The X360 slows down further in 2011, and MS brings out a new console later in the year, sells 3 million. Sony don't announce a new console till late in the year, due to come out mid 2012, so the PS3, while still slowing down with age, doesn't drop like a stone... sells 7 million.
end of 2011 gap = 1 million.
The new xbox starts taking over in 2012, but some people still look for bargains, x360 sells 1 million. The PS3 does have a successor coming out this year, but it still has the first half of the year to sell well, and sells 5 million.
end of 2012 gap = 3 million in the PS3s favour.... PS3 goes on to sell a further 2-3 million in 2013/14, while the x360 dies in 2013, selling off the rest of it's stock (~500k)
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I am not saying that it is going to happen, or even likely... but it is cetainly possible.
I think the lifetime sales of the two consoles will end around 45-55 million for both of them, I suspect the X360 will remain ahead by a few million, but it is not impossible for the PS3 to pass it.
end of 2011 gap