Are your scales on a hard floor! if you have them on carpet they will not read correct, usually under. Go way of boosting your ego, use the scales on a nice pile carpet.

Are your scales corrected for local gravity? Gravity varies in different places, even within the same country. Call up the weights and measures people tell them you need a local gravity figure to correct the scales whilst you stand on tiptoes with no clothes. I am sure they will be helpfull.