Are you by any chance using a remote start on the car? I have noticed that that when I use the remote start functionality (factory) that it does not add that time to the running duration and gallons used. Before Winter and remote starting every day I did notice it would be off a little at most fill ups, but usually not more than half a gallon. Also it is possible to pause it by pressing the center button on the left side of the wheel which will cause the gallons and the rest of the data to be tallied for that trip.
