For the "Determination of the formula of a compound from experimental data" lab, i had to add nitric acid to a tin

For the "Determination of the formula of a compound from experimental data" lab, i had to add nitric acid to a tin

foil and heat the solution until oxide was the only remaining product. From my calculations based on the data, my experimental % is lower than my theoretical % (77.1% < 78.8%)and my %error is 2.16%. How can that be explained and what are the possible reasons for error during the experiment?