Hydrochloric acid + baking soda experiment?

Imperfect dilutions and measurements of the substances would be the main reason for both. If the concentration of the HCl was under what it should have been, there would be less NaCl. If not enough NaHCO3 was used, the same would occur. If HCl and NaHCO3 were both used just a little too much, more would be made. Experimental error is what happens to everyone at some point, no matter how hard you try.