gta4 and price cut are saving the 360 for now. Good move by microsoft, at the end

It is a rounding error. Any time you round you create a certain amount of inaccuracy within the data that will eventually show itself. Using full numbers is often impractical, if not impossible though, so you have to choose a spot to round that will not significantly affect the end results. ioi clearly feels (and I agree) that when keeping track of market share you only need one decimal place before rounding. Consequently you can get an odd combination of numbers that will result in a slightly higher (or lower) number than expected. For example if the numbers were 33.45, 33.45 and 33.1 that would round to 33.5, 33.5, and 33.1 which adds to 100.1%.