The reason that "overall attach rates for multi-plat games are better for the PS3" is because there are fewer hardware units out there.
The fewer the h/w units, the more narrow the userbase is, and the better attach ratios are to various games. People that own X360s own vastly more games, from a much bigger library. Because of that, you can't have the same percentage of users buy a certain game when the base is so much larger. In this case, however, what matters is the actual sales of the game(s).
Does it matter if the PS3 version of Call of Duty 4 had a higher attach ratio? No. The X360 version has sold about 1 million+ more copies. That's all that Activision cares about: actual sales. A software company cares about sales, not percentages. If software is selling better for them on a specific platform, it's selling better on that platform.
Back from the dead, I'm afraid.
