What is it that the residue has smaller mass than original sample?

The residue has a smaller mass than the original sample because during a chemical reaction or a physical process, some of the original material is converted into gaseous products or other byproducts that are not retained in the residue. For example, in combustion, substances like carbon dioxide and water vapor are released into the atmosphere. Additionally, losses can occur due to incomplete reactions or evaporation. Thus, the remaining residue reflects only a portion of the initial mass.