Readings tend to be a snapshot in time. It will depend on a number of factors including what devices are running or powered on at that time. Conditions vary and can be impacted by power fluctuations as well as what the neighbors have running on their systems as electrical systems are actually interconnected. To test its effectiveness, try measuring it with the breaker off and with the breaker on. On rare occasions, the Power Perfect Box Wire-In may have been installed incorrectly. A common problem has been that the Neutral wire from the box was installed on the Ground bar rather than the Neutral bar in the distribution panel.