Like any electrical component hour meters can become defective but not broken, and record an abnormal number of hours. When was the last time you verified the precision of your hour meters? if they are not reliable the flow or cumulative volumes are based on defective equipment. If the hour meters are operating properly how exact are the pump capacities used in the derived flow formula? A difference on 10% of capacity between 2 pumps, could cause one of them to run 50% longer than the other and it could be normal. How do you know that the difference in run times of the pumps is normal or abnormal without knowing its present flow rate? In today’s technological world, using hour meters is like using horses vs. cars.