I had a tech explain to me that this is NOT a direct reading guage. Instead, it averages the fuel level over a period of time. How much time...I don't know. I do know that with my previous F-350 and current 3500 when I switch from Aux tank to Main tank that the needle takes several minutes to come up to the new fuel level.
It is logical that the system has a memory that can be lost if battery power is removed for a period of time. That being said, when you regain battery power, the system has to start from a 0 memory level (fuel warning light) then gradually average out with the current (35 gallons) fuel level.
Clear as mud...right