There are several reasons.

1. The light comes on 1-2 gallons before running dry (It would be horribly useless if it didn't). So you'll have a little gas left when you refill.

2. Just because the pump shuts off doesn't mean the tank is filled to the max. Pumps turn off when they sense enough pressure from the tank from it being filled to then turn off. But not all pumps are the same and some will shut off sooner and others later.

3. Heat. The temperature outside effects points 1 and 2. The hotter it is the more the gas in your tank expands. Now add to that the heat from the vehicle and you can have some pretty mixed numbers.

i.e. A cold car gas tank will have slightly less volume then a hot car. The heat from driving a car will expand the fuel. A cold morning vs. a hot afternoon will vary the volume of fuel. Pump #4 and pump #7 will vary slightly in how much fuel they'll dispense before automatically cutting off.

Ultimately you should monitor your mpg fuel consumption. Use your trip odometer to see how far you've gone on a tank of gas, then record how much fuel it takes to fill up again distance/gallons = mpg. Because your overall MPG matters more then if you have a 10 or 11 gallon tank.

For me my car has a 14 gallon tank, gets about 30 mpg, and gets about 360 miles before the light comes on, which means I have about 2 gallons left.
Source(s):
http://www.lifeaftercoffee.com/2005/08/08/how-does-the-gas-pump-shut-off/
http://www.realtechnews.com/posts/3428

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