gas tank only 13ish gallons???? missing 3-4 gallons

Do not feel bad......On EEEEEEeee, I cant stuff more than 13 gallons in my 1974 Dodge Dart Sport.....and I drove from Texas to Oregon, never able to put more than 12.5 gallons in driving on the highway from Full to Empty.

I do have a aftermarket 3/8" stainless Ebay sending unit......in the car...because I questioned the accuracy of the sender that was in the car when I got it....(turned out LATER to be discovered the Voltage Limiter in the cluster was Wiggy)...and I still have the Pentastar PN OE sender that I took out of it....It is definetly shaped differently.

It could be that our crappy aftermarket reproduction Sending Units....are standardized to fit all tanks....and cant suck up the last 4 inches of fuel in the tank...
OR that the float itself on the sender leaks and gets fuel in it and still floats.....but not properly.
Whos knows....its inside the tank....and I hate messing with Gasoline, and things that are not leaking.

I always believed A-body fuel tanks were 16 gallons too....