You know that, when you run a report, it is possible that some calls lie partially outside the report range; eg if you run a report from 9.00 to 11.00, a call starting at 10.59 and ending at 11.03 will be partially outside the range, as it will be "closed" by QM at 11.00 sharp (but marked as ongoing).
Theuy are usually a minority, so QM will compute stats for both cases. The average will usually not be very different, so it is normal that averages don't vary much.