That's because at the bottom of the tank there is a layer of charcoal for filtering that will hold about half a gallon, plus the fuel in the lines hold about a gallon and a half on an American car/truck. Plus there is a slosh value. Needle guages aren't all that accurate either.

(Those values were given to me by a physic's teacher with a Corvette, your milage may vary.)

I can hear the whine of my fuel pump at the tank, that's when I know it's time to hit a gas station