Fuel Tank -16 gallons.....why can I only put in 10 gallons?

Ok.....on my 1974 Dart Sport....Since I got the car...I have never been able to stuff more than about 9.5 gallons of fuel in the tank.

I figured it was a bad/worn sender...and ordered a new Pickup/Sending Unit....got the 3/8" pickup one....ran the fuel level down till the gauge showed empty and pulled the old sender and installed the new one.

Same Issue....its not a HUGE concern....but I sure would like to be able to fill up with more than 10 gallons for Highway Trips.

I cannot figure out WHY with a 16 gallon tank....I cannot fill it over 10 gallons.

I do not know of a smaller capacity tank...and I did not install this fuel tank....does such a thing exist?

Or what is the possible answers to this issue?