At the risk of being labeled a flamer...
The listed fuel capacity may not be what you actually can cram into the tank.
For the purpose of this discussion I'll ignore the various warnings about overfilling the tank.
If you want to REALLY know how much fuel you are using you need to find a way to fill the tank up the same way and to the same level each time.
Same way: Lets say on a level surface, or at the same filling spot pointed in the same direction.
Same Level: Can you observe a place in the tank or filler neck to fill up to? (Don't forget to burp the tank also to get the air out.
Eric