Not sure what “ROP” stands for, and therefore I’m not understanding the question, but here goes:
Maybe you know this stuff already, but the differences in your volt meter readings could be a number of things. The pressure variation from the meter lead to the metal, a bit of corrosion on the metal surface under the meter lead. To get a consistent reading, use some alligator clamps on the metal. If there is water or saline solution involved they will affect the measurements too, better connection/conduction.
ROP = Reversal Of Polarity. I see. The voltage should be the same either way.
ROP is Ring of Power. Someone was selling these thnigs awhile back and that was the name they used. At least I think that is the name they used.