Sense data provided not accurate

One of my customers who I installed Sense for got an email:

Sense found a new device and named it ‘Device 2’. It just turned on!

Looking at his data Sense provides the following information:

average runtime 28 minutes

Lets look at the log file:

Turning off in the same minute that it turned on ? That is not the same as 28 minute avg run time!

With the given contra dictionary information it is hard to identify the device