Why does the year difference matter? Zed's chart was brought up to directly contradict a certain assertion with data he's accumulated. We should know it by now, I think it's brought up every NPD thread.
I can't speak for him, but I suppose a possible theory is that XB360 owners, having had their consoles for a year longer, will have amassed quite a collection of games and may be less inclined to purchase more titles. Since the above table shows ratios of userbase:sales of titles, much like one would expect the 360 "attach rate" to be lower, this table just naturally shows that the 360 owners:title ratio is more diluted.
Of course, another possible theory is that more and more 360 owners have become multiplatform owners (like myself recently, having purchased a PS3 after the Slim was released) and are more keen to get more titles on that -- although, admittedly, I still only get multiplatform games for my legacy system.