Why does my Oscilloscope measure a different voltage when I change the vertical adjustment?
I actually texted Rigol and this was their answer:
&quot;The measurement accuracy is driven largely by the range setting. In  this case, you would get more accurate by zooming in on the vertical scale even if you moved the offset until the ground was off of the display.
The limitation is the number of bits per division on the display is more or less fixed, so you zoom in vertically to reduce the quantization error.
You can also use the High Resolution Mode in the Acquire menu. This is especially useful for signals like this because it allows the instrument to eliminate quantization error by averaging between the bits. So, you should definitely try this even before you spend time optimizing the range. It may erase much of the difference.&quot;