NaOH is used to titrate with the impure KHP to find out how much KHP is in the impure sample. Since the amount of NaOH consumed by the titration is directly proportional to the amount of KHP in the sample, then an erred reading of a smaller value for the amount of NaOH would translate to an under-measurement of the amount of KHP in the sample. This means the % purity of the sample will be mis-calculated as a lower value.