When I measure a square block such as the one your have, close the vernier jaws along the entire surface, from corner to corner.
That way you see any undulations and a more actuate measure.
By the way I would suggest your second photo measures about 19.85mm but without seeing all the digits, not sure. Except to say that it is not 20.00mm
But at the end of the day your 3d printer is calibrated quite well.