I wouldnt trust any oscilloscope to give an accurate voltage reading, (well.. maybe one worth $20k +)
Depending on the settings on the oscilloscope (volts/div and probe setting) 10% error might be perfectly normal.
Remember scopes were designed to graph changes in a signal over time, displaying the voltage digitally on the screen was something that was tacked on later, it wasnt what they were designed for. With digital scopes the scope cpu only has a limited number of vertical bits to work out the voltage from.
The more of your signal you have on screen (number of divisions between the negative and positive of your voltage) the more bits are used and the more accurate it can measure the voltage. The rigol has 8 bits vertically, like most scopes do, so you don't have a huge number of bits for accurate vertical measuerment like a DMM.
If your scope is setup so it could display 200V from top to bottom division and you're trying to measure 10V then your spreading the 8bits over 200V, not many left in the 10V area to measure your signal. Having the scope probe set to x10 also makes things less accurate but you do get the ability to measure higher voltages and it adds some protection to the scope so its a tradeoff.
So the voltage display readings on the lcd are going to be most accurate when the probe is on 1x and the volts per div results in your signal taking up the whole lcd.
In another thread where someone was having a similar issue i did some tests on my scope.
This was with a 5.00V signal
1x probe | 50v/div = not possible | 20v/div = not possible | 10v/div = 5.4V = 8.3% error | 5v/div = 4.99V = 0.2% error | 2v/div = 5.08V = 1.6% error | 1vdiv = 5.05V = 1% error |
| 10x probe | 50v/div = 6.63V = 32% error | 20v/div = 6.00V = 20% error | 10v/div = 5.73V = 14.6% error | 5v/div = 5.02V = 0.4% error | 2v/div = 5.01V = 0.2% error | 1vdiv = 5.03V = 0.6% error |
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If you want more info here is the thread
https://www.eevblog.com/forum/index.php?topic=1989.0