Regardless what potential you are using as your reference (neutral is often used as 0V reference in the context of line voltage AC) the voltage on hot and neutral will be different (by about 120V in a North American household).
Any current coming from the power source (utility, generator) through the hot wire to the load needs to somehow return to the power source to complete the circuit. The normal return path is the neutral wire which carries the exact same strength current in the opposite direction. In a ground fault situation a portion of the return current takes an alternate return path (ground wire, water pipe, moist soil, ...) which causes the current in the hot wire to be different from the current in the neutral wire. Even a slight difference in currents is sufficient for the GFCI to trip.--
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