Going on a memory of long-ago reading here, so this may not be gospel:
RNR = men whose normal functions took them to sea full time, served as a reserve to the RN in times of crisis.
RNVR = Men whose normal activities were ashore, but having a long term interest in boating (an expensive middle- to upper-class hobby) Also served as a reserve to the RN in times of crisis.
I believe that both groups generally went into the wardroom, the ratings coming more from the conscripts, and of course the regular RN.
Wisdom of the era consequently had it that the RNR was sailors trying to be gentlemen, whereas the RNVR was gentlemen trying to be sailors.