Basically, the claim is that many multiplatform games sell to a higher percentage of PS3 owners than Xbox 360 owners. In other words, the 402,919 copies of Street Fighter IV sold on PS3 represents a larger percentage of its installed base than the 446,435 sold on Xbox 360. Thus, the attach rate for the PS3 version is higher, even though it's sold fewer copies. Sony also cites Metacritic data in this campaign, noting that PS3 has a higher percentage (there's that word again) of titles with a score of 80 or higher than Xbox 360.
The elephant in the room, of course: Microsoft's console has a larger installed base -- by about 7 million or so -- and a larger software library, tilting the odd percentage game in Sony's favor. There's a valid point to be made here, specifically that certain multiplatform game sales are faring very well on PS3 despite the console's smaller installed base. You just have to unwind the spin to find it.
