If you think about it, at this point in time who exactly are buying these machines? People just getting into gaming, and people buying replacements/upgrades - both things that would (I'd imagine) be affected very significantly by the generation up-to-now.
What I'm getting at is 360 *should* by all rights be outselling PS3 by some margin as its sitting on a very significant install-base lead at this point, that advantage giving them better "word of mouth" (in the broad sense that your peer-group is more likely going to have more 360 owners), and a bigger pool of upgraders to draw from.
Essentially, if the 360 wasn't doing better than PS3 by a factor roughly equivalent to their install-base advantage, something would be badly wrong.
Looking at Sony's offering, to be truthful I can't see a real reason why their performance should be improving more than it is. Software-wise they put out good exclusives, BUT they are the same exclusives they've been offering all this gen. A highly rated Uncharted is still only going to appeal to the people who liked the sound of its previous entries...
Bottom line for me is that really nothing much has changed on the software landscape since the start of this gen; and as a result why would anyone expect the status quo to change.