The sensor is essentially a float in a wide(ish) tank.
If you just replenish with a small amount of fuel, the level in the tank will barely move.
If you notice in most cars, it seemingly takes ages to move from the full mark, but conversely also seems to vanish rapidly when you're down to a couple of bars.
As the fuel tank is not a regular and uniform shape, there are usually algorithms coded into the ECU which take the signal from the sensor, apply a bit of maths to it, then send a calculated value to the display.
It's normal practise to apply a more pessimistic algorithm to the lower end of the fuel level capacity, as it protects the end user from running dry.