You do realize that in real life even "empty" tanks still have gas in them? The pickup/pump/drain doesn't get it all. There are puddles. Especially if the tank shares the dihedral of the wing, and the pickup is on one side or the other, you might have quite a bit of residual gas.
I recall a story once where a pilot almost ran out of gas in mid-flight, and when he landed they checked the tanks. There were only so many liters left in each, and the tanks read as empty. The pilot was in disbelief that the engine hadn't cut out.
Just means "usable" fuel isn't always the same as "all the fuel that's in there"
EDIT: You ever pour yourself a drink from a picnic thermos? One of those round or square boxes or barrels with water and ice inside, and a spigot at the bottom? You put your cup underneath, press the button, and get a cool drink. Ever try to get all of it out, when it's empty? You have to twist and turn the thing at odd angles to get all the drops out, and even then you miss some. I'd imagine something similar happens in fuel tanks.