The fuel gauge is controlled by the Instrument Cluster module based upon a resistance value (ohms) from the fuel level sensor in the gas tank. There is a min to max range. The resistance readings are not immediately actioned to the fuel gauge, there is a delay which prevents sloshing fuel or car angles from causing the fuel gauge to move erratically. So with the same fuel level having the car parked at different attitudes would change the fuel gauge readings.
Perhaps you could explain your symptoms a little more.