FFXIII sales aren't that amazing, it's more or less on expectations, right? This week was abnormally high, but it was the New Year's boost. I mean, even Tales of Graces (that bomba) went up to ~18k this week. (...I hope that's accurate!)
Anyway, this is the only time we'll see the PSP above the 360 for a long, long time, I'm guessing. In early January, Japan has an abnormally higher effect on WW hardware than usual, so I expect the PS3 > 360 gap to slowly come down in the coming weeks as well.