the most I ever squeezed into my tank was 7 gallons and I always wondered the same thing. I got nervous about the gas gauge possibly being off so I just always reset my trip odometer when I got gas and refilled every 350 miles or so. I did notice the bottom half of a tank of gas seemed to go quicker than the upper half of a full tank. I thnk the tanks are just shaped weird or the sending unit is in an odd place or something.
By the way we LOVE pictures of members' cars, no matter what shape they're in, so post some up if ya can