Because it has a fuel level meter (you have it even in very old cars but in the analog version)? A small and simple mechanical device checking on which level is the top of the liquid (fuel). This is the same device that signalize the end of the tank extended with a small  computer that can compare and store previous and actual level

If I know how much the fuel in the tank was before refill and I compare it with the level I have now the result is obvious.

I remember cars that have no such float support- the fuel level check were done by the stick... long time ago in the far far away country 

Its a car computer and can detect anything and everything that goes on in your car. So when you add fuel it will detect and show you. And it will also inform you on the amount of fuel that's remaining and what you have used.Moto Metal Wheels