Why do glucose meter results vary up 50 and 80 points when tests are done less then a minute apart?

My son is Type 1. We are having significant variations with the glucose meter. For example, he gets a reading of 21, retests within one minute and gets a reading of 75. Sometimes we retest using two different meters. Anyone else has had issues like that? It happens several times a week. I even called the manufacturer to make sure the meter is working properly.