RMS is not a real value.. It is a imaginary DC value that would give same thermal equivalent power like the complex waveform you are measuring, if you were to connect them to the same pure resistor (no parasitic inductivity or capacitance).
So if you have same sinusoidal waveform with and without DC component, it will greatly differ... Real TRMS is AC+DC.. AC only is not really RMS, except for signals that have no DC component, in which case it is the same... Many TRMS voltmeters don't have AC+DC, only AC RMS, which is not really true RMS measurement.. It is ok if you only want to know AC component...
If you are measuring with the scope, and use DC coupling you are measuring AC+DC RMS, with AC coupling, also only AC RMS... So it is easy to have mix of instruments that are not measuring the same thing..
Also, you have to keep signal within instruments AD converter range.. so 200mV on top of 10V you measure on 20V range if you are doing AC+DC, or you can measure at 400mV on AC only...
You have to understand what are you measuring... That is why we have oscilloscopes, voltmeters alone are not enough to characterize unknown signal..