Possible a "false ground". This is when someone decides to replace a 2-wire receptacle with a 3-wire grounding type, and there is no ground in the box. They jumper the neutral to the ground terminal of the receptacle. Or, try disconnecting the individual branch ckt. neutrals at the box, then turning the breaker back on. If there is a change in your readings, or, if the circuit is still functional, this would point to a parallel neutral path.
