The Spooky settings 20V p-p - and 4V p-p - is peak-to-peak value, it is the range between the maximum and minimum.
Such as sine wave, the wave crest to wave Hollow is -10V to +10V, so peak-to-peak value is 20V p-p.
The oscilloscope and a true rms meter can test output signal, but the measurement frequency is fixed 50Hz.
You could test out RootMeanSquare by using rms meter, it is not able to test out peak-to-peak value.
The voltage amplitude is correct.