I suspect that is because gas tanks aren't necessarily the same shape from top to bottom. The amount of gas is measured by a float in the tank, so the gas gauge in the dash is reflecting the level of gas left, not the volume. If the tank is curved at the bottom or has an indentation or narrows the level is going to go down faster toward the bottom of the tank as the smaller volume of gas gets used up.

There are many threads on this subject, I even started one myself a few weeks ago about "most miles on one tank".
http://www.greenhybrid.com/discuss/f...-gas.9629.html

The TCH gas indicator hits empty with 0 cruising range at about 14g, which means you have about 3g in reserve. Depending on the mileage you are getting, you have another 90-150 miles in the tank. I am getting about 600-610 miles on each full tank, and it takes me about 16.5 to fillup. There are people here averaging close to 700miles on each tank. And one poster "nash" is getting such good range that his cruising range indicator has adjusted itself to show 660+ mile cruising range (see above link)