With an install base which was more than both new platforms combined.
Well FFX sold million or so more than FFXII with less than half the install base, baffling isn't it?
The install base argument doesn't always hold true, especially for first next-gen sequels on new platforms (GT3>GT4, MGS2>MGS3, FF7>FF8>FF9 etc). FF13 is the first one, guess it'll also be the highest selling one till the next-gen sequel.
Gaming PC - i5 4570, Asus R9 290 DirectCU II Liquid Cooled with Kraken G10+H55, Ripjaws 8gb ram, NZXT Phantom 530 Full Tower, Corsair 650w