Skyrim also had/has (no idea if its fixed yet) some wicked bad glitches for the PS3 glitches that would ruin it's playability later in the game. I wouldn't be surprised to see the 360/PC versions a bit higher.
I'd actually bet the 360 GTAIV numbers are lower - more active online community, RROD issues.
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Donny: Are they gonna hurt us, Walter?
Walter: No, Donny. These men are cowards.