The way fuel is estimated by the car is based on readings which change,
the amount of fuel available, the way you have driven it before, the temperature.
also, the way fuel in tank is measured is either by one float chamber, or in the case of saddle tanks, two float chambers, rather like your toilet cistern.
these are not accurate and are not designed to be, when you drive up a hill, the tank reads different to driving down the hill.
fuel rank senders can fail or stick also.