My yamaha keyboard saves midi files to the 0 format. If you saved it in pt as a 0 format midi it makes sense. If you saved it as a type 1 midi then I would think it would play on seperate channels. My old PSR 2000 does not have a usb, so I'm not sure what would happen with a memory stick. Maybe it automatically converts any midi to a type 0 for keyboard recognition purposes?

Just a guess