The problem is the voltage polling on Zen is too fast for Windows to read, CPU-Z is even slower. RM reads polling for the clock speed and voltage as fast as Windows will allow, but it can't give you the right voltage at the exact time, even if Windows could read it its far too fast to see.
See how CPU-Z thinks i'm running 1.44v at 4.2Ghz, RM is reading "Peak Voltage" 1.426v with various cores at different speeds, the highest being 1.55Ghz, "Peak Voltage" is an important distiction, its the highest voltage at that given polling period, RM was showing 1.44v a split second before but it read a change in the volts and clock speed before CPU-Z. its why they show different readings at different times.
RM is the best monitoring tool for Ryzen.