Blindjim,
A difference of potential, voltage, measured at an electrical outlet measured from the neutral to the equipment grounding conductor can be the result of VD, voltage drop, on the loaded neutral conductor. If the resistance of the equipment grounding conductor is less than the resistance of the loaded neutral conductor then there can and will be a difference of potential between the two conductors.
VD is directly proportional to the size, length, and load placed on the conductor.
Poor connections along the length of the conductor can cause higher resistance in conjunction to the amount of load placed on the branch circuit neutral as well.
As Simply_q said in his post the main service earth ground connection does not come into play. In most cases the branch circuit neutral conductor as well as the branch circuit safety equipment grounding conductor terminate on the same neutral/ground bar in the main electrical service panel.... Sub panels will have them separated on two different bars but they still end up tied together at the main service electrical panel via the feeder neutral conductor and equipment grounding conductor. Both connect together at the main panel.