Most float type fuel tank level sensors aren't designed to be super accurate, so it throws out whatever display you use.
In a typical rectangular tank, climbing/descending a hill, cornering, road camber, how warm/cold the fuel in the tank is and probably lots of other outside influences will effect the displayed fuel level.
Some type of damping device is usually used to smooth out the reading, but they aren't perfect and will give false readings at times.
An MPGuino is pretty accurate, but even that ignores variations in fuel pressure and temperature, dirty or worn injectors, fuel leaks, etc.
This is why most guys recommend only filling from one particular pump at one particular petrol station at a regular time of day. Even then, there are just too many variables to rely on one tank's result, you have to average results over time.