I believe Ltuned is right.
The tank holds about 65L. From what I've read, 55 is your main and the other 10 is the reserve.
The light comes up when the main is close to 0, but your reserve sits without notice. Sounds about right as I've gone done to 20KM remaining, which I know is an estimate. However I've never been able to do a full 65L refuel, only ever 53-54.
I could be wrong, but sounds plausible 0.o