1024 looks like the number of edges per revolution. Assuming you are counting only rising or falling edges, that would make your encoder to be 4096 steps/rev, which is pretty standard.

Where does 60 come from, I have no idea. Do you have any sort of gearing between the motor and the encoder?

In your case, if you count N pulses per minute, then RPM = N / 1024, since 1024 pulses constitute the full rotation.