Off by 7V at 100V/div? What do you expect at 100V/division?
Every pixel will be like 10 volts, and the noise will do the rest. Normally you adjust the scale to the signal voltage.
The scope has to scale the input, and ADC the precision is just 8 bit. Only high-end equipment($$$) will have better ADCs.
There're 8 vertical divisions, so:
8*100V= 800V, divide that by 256 and you get 3.1V/bit.
But it seems the scope actually fills 10 divisions, so:
10*100V= 1000V, /256 = 3.9V/bit.
1-2 bit error is something expected on this scope, lower noise belongs to the expensive market!
Use the correct tool for the job! A scope is not a DMM, its function is to display waves.
Even most DMMs will have poor accuracy in higher ranges.