you know how much gas you put in.
Again, you are assuming that the amount of gas you put in is exactly the amount that you burned up. And that isn't the case necessarily. There are factors that affect that - like temperature, pressure of the pump and sensitivity of the pump, vapor in the tank ..... so while yes you know how many miles you traveled, and yes you know how much gas you just put in, you don't know if that is an accurate reflection of how much gas you burned.
So it is all just an estimate and no more accurate than the computer in your dash or the piece of paper in your hand with your hand calculation on it.