Scales seem like pretty simple, straightforward devices but actually, all kinds of little things can give you an inaccurate weight reading. \
First, make sure the scale is on a perfectly level spot. The scale should also be flat and not sloping or inclining in any way. \
Also, make sure the scale is set to zero before you weigh something on it. If you do these things and you're still getting different readings from the scale, it may need calibration. \