Measurement error over 19.6V

If I take measurements and want to show the voltage, then I can only measure up to a maximum of 19.6V. Everything over it, is no longer displayed.
It does not matter if I measure DC or AC.
The Oszi should supposedly be able to measure up to 30V? What can be the error here?
I would be grateful for every tip

The maximum voltage to display and measure is +20V and minimum -20V.
For safety reasons the max input voltage is specified to 30V.