[U.N. Owen]

My theory is there are more vertical 'frames' that the notes must traverse in stepmania as opposed to ffr. Let's just say there are 45 'frames' in ffr and 54 in sm. If you play 2x on ffr (translates to C620), then you would have to multiply this number by about 1.2 (54/45) to result in a stepmania speedmod of around C744. Just my guess.