You need to include all games. What you have is a view on the games by exclusives (not a bad analysis) but without a full analysis (all games available) I don't think it makes sense to apply the results as indicative of demographics - it's indicative of exclusives really.
Also, should MGS be a shooter and Dead Rising and Crackdown action? Don't all three let you shoot? Although they are clearly different they're all essentially violent action titles and hardly represent huge demographic differences.
Personally I don't think the 360 is all about shooters, I do think they will sell more on that console in US vs other consoles/regions as it is very ingrained there vs PS3 with FPS/online fans - but that's like saying the sky is up I guess.
But reality doesn't matter in a way. What's more influential is market perception. If the 360 is perceived as a shooter console most general buyers will assume its true.
And I'd say that in for many people the 360 is still seen as being all about shooters. Unfair? Probably. But the Xbox ended up with that stigma for a reason (Halo) - its up to MS with a constant stream of diverse titles and marketing to change the general opinions kicking around. I agree with you its annoying when people say it - but they're not going to stop until the brand perception changes. That's the same for everything.
Try to be reasonable... its easier than you think...
