The difference comes from the fact that dissolved nitrate is expressed in mg/L N, and the calculated value is converting the results to mg/L NO3.
For example, sample ROB-10:
When NO3 (as N) result of 2.2 mg/L N are converted to NO3 the molecular weight will be used:
Dissolved NO3 (as N) * molecular weight NO3/molecular weight N = Dissolved NO3 (as NO3)
2.2 mg/L * 62.0049/14.0067 = 9.74 mg/L