If the Block history is selected, the RMS value will be calculated for each block size separately.
The block size is defined with the FFT resolution and the sample rate.
This is how the RMS signal (red curve) looks like:
If the Overall (Averaged) FFT is selected, the RMS value will be updated with each block.
The RMS value will not be calculated for each block individually, but it will be calculated from all the previous blocks and the current block.
You can see that the RMS result is jumping less and less with time if you have a similar signal like in the picture below.
That is why the last RMS value in the data file (go with the yellow cursor at the end of the data file), will be the same as RMS single value.
If you need a single value RMS number also an RMS marker can be used for that purpose.