The only way for the voltage on one leg to increase (absent a transformer of some sort) would be for there to be a poor connection on the neutral line somewhere between the pedestal and the coach. In that case, the leg that is more heavily loaded will see a lower voltage and the other leg a higher voltage. If this is happening, definitely get it taken care of.
I would tend to suspect instead that the calibration of the two units is slightly different. I think I read that there's a calibration potentiometer on the circuit board for the PI EMS units that a technician could use to correct whatever error there is. (Other brands may or may not be adjustable.) However, I have no reason to suspect that either one is not within spec for calibration; these are not laboratory grade instruments, and they do agree within a percent.
