Joker Challenge Maps.
Just goes to show that there are several hundred thousand (if not many more) multiplatform gamers out there that are willing to switch platforms for one release vs another if there is enough incentive to do so. Given that the 360 is so popular in NA, I'm sure multiplatform gamers generally favor the 360 for multiplat releases, because of XBL, achievements, etc.
This is probably why FFXIII has more pre-orders on ps3 in NA as well. The ps3 version being on a single disc, having slightly better graphics (or so IGN says), and the fact that Versus is still exclusive (for now) has probably lead many multiplat gamers to go with ps3 for XIII, when they'd normally go with 360 otherwise.