I'm not sure why you are getting the differences, but some of the more common causes of differentials are; the water sample is not drawn from the same place and depth in the pool, the drops are not the same size, inconsistent swirling, not enough powder, not giving the proper amount of time for the chlorine to distribute since the last addition.
I suggest to not add any chlorine today and retest tonight and tomorrow morning. Make sure everyone is done swimming at least an hour before you take the sample tonight so the chlorine isn't working on anything extra from the family during the test. Use a 10ml sample and test only one time, trust your testing.
I suggest to not add any chlorine today and retest tonight and tomorrow morning. Make sure everyone is done swimming at least an hour before you take the sample tonight so the chlorine isn't working on anything extra from the family during the test. Use a 10ml sample and test only one time, trust your testing.