I think technically speaking there could be a delay between two points. However, the "hum" being fingerprinted is the precise frequency which is tightly coupled through electromechanical action of all the generators on the grid spinning collectively at the same speed.
Im sure the waveform wouldn't be perfectly matched from one generation area with another, but when you look at a longer period of time, say a few seconds, you wouldn't be able to refute the waveform match.