Answer:
The presence of the contaminant increases the total mass.
And on continuous heating to remove the volatile impurities, the hydrated salt gets overheated and the anhydrous salt gets thermally decomposed thereby producing a gas.
The heating leads to the formation or release of a gas which leads to loss of a higher percentage of mass from the compound making the mass percent of water in the hydrated salt too high and the mass of the anhydrous salt remaining in the crucible too low.
Hydrated Salt ⇒ Anhydrous salt + Water hydration