The scale measures the mass of a substance and density is mass per unit volume. If the volume is already measured, then ignoring the weight of the water in the pan would decrease the mass being divided by the same amount of volume. This would result in a smaller value for density. For example:
Mass of mineral + water = 10 g
Volume = 10 cubic centimeters
Mass of mineral alone = 8 g
Before removing water, the density would work out to be
10/10 = 1 g / cm^3
However, if we remove the water,
8/10 = 0.8 g / cm^3
