Two lines of thought. 1) Back in my days at the dealership (mid 80s) the difference between a 26 gal tank and a 30 gal tank was determined by haw far into the tank the filler tube was sank. If the filler tube was, as an example, 6" into the tank when the fuel reached the end of the filler tube, it was full, no more fuel would go into the tank. If the filler tube was 3" into the tank, another 3" of fuel could be added, making it a "bigger" tank. Its possible the filler tube is too deep in the tank, may it have slid down deeper, or was always one of the "smaller" tanks that has never been checked against empty. A partially plugged tank vent hose will do the same thing.

2) Maybe your pickup tube inside the tank has broken off, cracked, or developed a leak a few inches off the bottom and when the level reaches that point, it sucks air instead of fuel, and will not let you use all the fuel in the tank.
Gene