PS3's "AAA" titles are simply not as big as the X360's. People seem to put Resistance up against Gears Of War, and it's ridiculous. If both were 360 exclusives, Gears would still smoke Resistance in sales. GeoW at one point had over a 40% attach rate - it managed 4 million sales when the 360 userbase was only 10 million strong. A game cannot pull this stunt off just from good marketing.
The problem with Sony's games is they aren't very mainstream, or standout much from the crowd. I think Resistance and Motorstorm only did well because they didn't have much competition, and now that the real games are out (CoD > Res), their time is up. I'm not debating quality, just general opinion.
Valkyria Chronicles sold "low" for similar reasons other great games like NMH, Zack & Wiki, and Tales of Vesperia did. Niche Japanese games don't tend to sell all that much, no matter how high its metacritic score might be.
LBP is the exception, and I think it's doing fine. The game was never supposed to be frontloaded (like all of Sony's games). The 360 gamers' tastes probably just share more in common with each other than the PS3 buyers - it probably helps how ~70% of its userbase is from the UK/US (english speaking).
