I believe the weight tolerance is +/- 0.1 g, it could be dirty, and/or your balance is out of calibration.
The picture almost reveals as though there is a coating on the coin. Is it possible someone clear coated it?
The coin is very fine, and I have new jewelry balance, work very good I bought it month ago, and I tried to weight a lot of other 1983 and all of them 2.5 g only this piece of coin?