The other aspect to this is that when PS3 games sell its more often in Europe where the Euro is still much stronger then the dollar. This means that a game in Europe is worth 1.3 or sometimes 1.4 games in America. That is why EA makes more on the PS3 then the 360 most of the time, even when the 360 in some cases sells more games. This is also true of PS3 games selling more in Japan, again strong yen=more money per sale for games in Japan where Sony is also strong. Given the US treasury has made several trillion dollars by printing money over the past few months, currency devaluation is almost certain to keep this trend of a weak dollar going.
The reason the range has narrowed significantly (besides PS3 and 360 user bases getting closer together) also relates to four other aspects.
1) sold PS3s are more likely to be in use then sold 360s. You can say "OMG fanboy don't bring up RROD!" but it remains true anyway. Now that the warranty period has passed on many 360s, if one 360 dies and someone buys another thats one 360 that shows in console sales but not in increased software. If someone's 360 dies and they DON'T buy another, then the loss of software sales is even more significant.
2) 360 games used to have an advantage in that they had gamerscore points while the PS3 version had nothing. With trophies this has changed, so there isn't really a reason to get the 360 version for achievements in a lot of cases now.
3) Free online. I used to play some games on the 360 when it was new and I had live gold, but since getting a better PC/PS3 I've just stopped paying for live and played whatever game I want to play on the other two systems. If you can play Call of Duty on either, why buy the copy for the console where it costs another 5-10 bucks a month to play online especially these days where every dollar counts?
4) between free online and trophies a lot of two system owners seem to be moving to buying games for the PS3 over the 360. This can be seen with games like GTA 4 where the sales per console are very high on the PS3 vs the number of PS3's in the wild. Such a high number would be almost impossible without a large number of dual or tri system owners (which are almost certainly not a small part of the video game buying population) choosing the PS3 version instead of the 360 version despite the promise of later DLC.
As more people get PS3s in addition to 360s and trophies become more wide spread it seems likely that this trend of dual system owners choosing the PS3 will continue and even accelerate. This idea is backed up by the increasing parity of game sales between the two systems even in North America where the 360 has a 2 to 1 install advantage (but almost never anything close to 2 to 1 game sales on multiplat games).
That said the only way the PS3 is pulling ahead on most multi plat games in the next year is if the PS3 drops to 299 ideally with a packaged game. It's just not going to pull ahead as long as the ratio of consoles in the wild isn't closer. Sony needs to entice more 360 owners to pick up a PS3.
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