It is possible somehow to calibrate this scope to be spot-on ?

Mine is showing someting like 3,75V rms when it should be 3,5V rms at any frequency ( of course the rest of measurement , amplitude and so on are affected too ) . The error is the about the same at any input voltage and attenuation .
It is not the probe , it's the same with a simple cable BNC to BNC . The signal is measured with calibrated instruments , so there is no external error .
This was the same when it was brand new .
Self calibration was done of course numerous times , the firmware is updated to the latest version .