Packaged meats are often weighed once they're in the package, which is a no-no as it can add considerable weight to the final product. 

It happens most commonly with the kinds of foods that get packaged in a grocery store, like meat and fish and sometimes cheese.

The foods usually sit on a Styrofoam or cardboard tray or are put in a plastic tub covered in plastic wrapping, with a label that is supposed to specify the weight of the food product alone and what it costs per kilogram or per 100 grams. The weight is not supposed to include the packaging.

SOLUTION? Buy freshly sliced meats and cheese from the deli counter.