Fuel sender and gas gauge in ‘63 and ‘64 Darts.

On my two Darts, the gas gauges work but both prematurely retreat down below the empty line. They pretty much lay at empty after 100 miles or 7-8 gallons, so I know there’s 10 gal more to go. Then the needle will eventually lay down below the empty line and At that point, I’ve determined, that they actually are still holding 4 gallons in the tank.
Is this common? Any ideas.
Thanks