In every chemical reaction ever performed, mass is conserved. If you start with #100*g# of reactants, at most you can get #100*g# of product. In practice you are not even going to get that. Reactions seldom proceed with #100%# yield, and even if they do, some losses are going to occur on handling, i.e. getting your product out of the reaction vessel, purifying it, getting it into a tared bottle; mass loss occurs down the line.
Here, Jim lost #(95-82)=13*g#. Jim was apparently a bit lax in scraping his product out of the reaction vessel; or maybe it didn't crystallize out of solution completely. If the product were pure, the loss may be acceptable.