My Chevy is about the same. I have run it to empty and it only takes 19 or so gallons in a 26 gallon tank (actual fill is generally 25 gal unless I dribble it in to the very top). Most of the time I fill at around 12-15 gallons or the 3/4 full mark.
I can read the Display and it gives an accurate reading of how many gallons I have used-if I remember to reset it :)
I considered the possibility that the tank had collapsed some amount (it is a plastic tank after all) but after checking as much as I could, without dropping the tank, I now believe the fuel level sender is just inaccurate.