Hi,

Due to solute solvent interaction, we obsereved deviation from true value.

Like negative deviation and positive deviation

As the value of colligative properties depends on the number of particles of the solute, the experimentally observed value of colligative property will be higher than the true value, therefore the experimentally determined (observed) molar mass is always lower then the true value.
for KCl (electrolyte) the experimentally determined molar mass is always lower than the true value when water is used as solvent.

Example with small deviation is Glucose 

Thanks