Thats because it is a thermistor. As long as it is cold (as in gasoline covering it) the light wont come on. When the thermistor is exposed to air, it heats up and the light comes on. thats why the light isnt a on/off thing. It slowly glows. I dont know what the calibration is for it, but i can see where in winter, if it is cold snough, the thermistor can be fooled that the tank is fuller then it really is.
There is about 2 gallons of gas left when the light comes on.
nipper