fuel tank only holds 9 gallons on 1999 Saab 9-3

gauge works, tank only holds 9 gallons. when fuel is low, gas warning light comes on.

Asked by for the 1999 Saab 9-3
9-3 sedan holds 16.9 gallons, if tank only holds 9 gallons the tank must be damaged or sender unit is damaged/out of calibration.
with the fuel light on, the max fuel the tank will accept is about 9 gal. the gauge or sender being damaged would not pysically stop fuel from being added to the tank, so that can't be it. In what ways could the tank be damaged? A cursory exam shows no dents or damage. does the fuel tank have an interior lining that could have collapsed? Anyone?