I think the Racing category is pretty even. Shift has low numbers because it came out around the same time as Forza 3. Otherwise the other multiplatform racers and exclusives sold about equal when combined. Obviously that'll change with GT5 :P.
I think the difference spawns from what's available on the consoles. The 360 has more WRPG's available. Therefore people who enjoy WRPG's are likely to grow their collection on that platform.
The PS3 has most of the JRPG's the 360 has with extra content as well as a few exclusive ones. In addition to the Playstation brand is pretty much known for it's JRPG collections. Therefore collectors of JRPGs would naturally migrate to the PS3 to continue their collections spanning back to the PS1 and 2 days.
Shooters are pretty even actually as they seem to be the top selling genre on either platform.
There are a couple of reasons for the difference in the fighter genre. First, the PS3 is known to have a more fighter friendly controller. Second, the games listed there are either new to the Xbox install base or are developed and styled towards an audience where the PS3 install base is much larger (ie. BlazBlue and Japan).
Hack'n'Slash is a combination of all the above reasons. The history of quality hack'n'slashers available on the Playstation brand, the intended audiences of the games released and the install bases of the consoles in those regions, exclusives of the same genre on the given platforms (360 doesn't really have any since NG isn't exclusive anymore), etc.
