Most, if not all, fuel gauges are calibrated to read empty even though there is still gasoline in the tank. The reason is because the typical fuel pump is lubricated by the gasoline (no oil or grease, right?!) and running to full empty is bad for the fuel system (pump, filter, and possibly injectors) because there's sediment at the bottom of the tank that gets picked up. By having a safety margin, automotive engineers created a means to have the driver get the needed fuel before running dry and damaging the fuel pump.
Conversely, a lot of fuel gauges are calibrated to read Full for about 20 miles before beginning to show any gauge movement because it's considered appealing to many drivers to not see the fuel gauge begin to go down after "only a few miles". This was more for marketing-type issues.