The gas tank is like most Asian branded vehicles. They state a size yet when you fill it up you are left scratching your head. You can trace that back to the gas guage.
I have a 04' Kawasaki touring bike and the gas tank/guage are the same way. At 1/4 remaining on the guage it seems I can fill it up and still have a few mysterious spare gallons somewhere. I think they overly cautious so that you have adequate reserve.
I was traveling with the Tundra and I got right above "E" before finding a filling station on the highway in upstate NY. When I filled it I put 22 gallons in. SO at "E" with the gas light I still had aprox. 4 gallons remaining.
My wife's Nissan has the Nav readout so it has a digital gas and remaining miles to go screen. It is also very cautious and tends to tell you to fill up even when there is a 4-5 gallon reserve left. I believe they do this to avoid pump/heating issues in the tank. JHMO
How do you know that it was absolutely empty?
The gas gauge on my F-150 is the same way. It can be a little bit below 1/2 on the gauge but it will only take 13 gallons to fill it up (its got a 30-gallon tank according to the owner's manual).
Its not limited to Asian brands. In snowmobiles, Yamahas have by far the most accurate fuel gauges of any brand (a lot of other brands of sleds still use the old mechanical gauges with the float in the gascap)