There's what's called a "full reserve" so it stays on F for a while (maybe a couple of gallons). The same is true for "E" - when it indicates empty, you have a little gas left. This is pretty common. The automakers want to have their cake and eat it too: they want to give the illusion of good gas mileage (first half often takes longer to burn because it's sometimes much more than a half) but they want to make it so customers don't run out either. Trucks usually have more reserve because possibility of abysmal mileage (towing, idling, etc).
I noticed you have a Sienna. So do we. Notice how its gas gauge works? It has about six or seven positions that it's always in, it doesn't act even remotely linear. 21 gallon tank, 3 gallons on F, about 4 gallons left when on E, stays close to F for a while, then slowly moves down. From 3/4 to 1/2 is pretty quick, then it gets stuck at the 3/8 mark for seemingly forever. After that, it rockets toward "E", and I've checked that there's at least three gallons when the gauge indicates completely empty. I drove on the light for 25 miles, then put in only 17.7 gallons or so. This means you can drive 100 miles on the light on the highway!