Way back at the panel, the neutral and ground at tied together...but, wire does have some resistance in it, and often, the ground wire is a smaller gauge (higher resistance). Throw in possible wire nuts, screw connections, etc., along the way, and there could be a difference in their potential. Probably nothing to worry about. It might be a less than perfect connection somewhere along the string.
If you were to measure ground-neutral back at the panel, it should read zero volts. If it doesn't, then, I'd start looking for a loose or corroded connection.