Most manufacturers hold a "reserve" of a couple of gallons in their tank after reading "empty". This is similar to turning your clock ahead 15 minutes so that you always arrive on time to an event, eventually you know that you moved the clocks forward and you adjust in your mind for it.
Another reason for the reason why manufactures leave fuel in the tank when it's reading empty, and I believe this started with Honda in the 70's or 80's (just not to the degree that it's done today), is because they don't want their vehicles sitting on the side of the road and the average person driving by will think that the car broke down, instead of running out of gas.