Different data on Power Chart range brush depending on zoom

I'm unsure if I've got a bug or not understanding how the information is calculated from the data. I'm looking at the history of a flow sensor. When zoomed out to a 15-day view, with the range brush looking at a 6-day window, I get a total flow of about 1720 gallons. When I zoom into a week view, and the range brush is selecting the same data (give or take a few minutes), I'm showing a total of about 3300 gallons. The difference in the two values doesn't account for the user error of a few minutes in manually selecting data with the range brush.