Hard to know what degree the guys that build these had, but I suspect not much experience with model railroading!
However, given the fact that the ZW-C puts out a sawtooth waveform, I wouldn't count on the voltage measurement being all that accurate. Hard to believe they are doing a true-RMS measurement in that meter assembly, so getting accurate readings of the sawtooth waveform probably isn't in the cards.
Use one strictly for building lights.
So 10 Volts on the meter may not be 10 Volts at the stud? Would it display higher or lower than the output?