Just a guess, but based on a (very) little bit of experience with a Rigol 'scope (MSO4000), some operations/measurements were performed **only on what was displayed on the screen**
. So if you have some non-integer number of cycles displayed, maybe it's calculating the rms value of what you can see. It might also explain the wrong frequency. It might also explain the 'too quick' rms calculation.
Or I might be completely wrong
I understand they fixed the FW in the MSO for the problems that I saw, and one would like to think that they learn from their mistakes...