Are these 'testers' analog meter variety? The cheaper the meter component the more inaccurate they are.
A analog meter is supposed to considered *most accurate at FSD (full scale deflection). Connect them to a good variable power supply and set the output to what the maximum range the meter should read. If that is anywhere near accurate, then check the meter at 1/4, 1/2, 3/4 range to see how good it is.
edit: most