Alex, the Rad Alt cert requirements are for a calibration so that zero is indicated at main wheel touch down. In most aircraft the oleo compression is relatively small and / or any difference between extended and compressed position is off set by de-rotation, and / or change of attitude with the nose wheel on the ground – all subject to aerial location.
Some Rad Alts give unusual indications on the ground when taxiing over water puddles due to double reflection of the radiated beam.