Looks like you ran into a bug somewhere along the line. We store your master average MPG in the database for quick retrieval, and that can get out of synch with your actual average MPG. I recalculated it and all seems well.
We've had an issue in the past with moving individual fuel-ups between vehicles--sometimes editing things can cause those numbers to be out of synch. If you remember any sort of editing you did that might help us track down that problem.