Gas pumps have within a range to get a passing grade , one of my jobs many years ago was to calibrate the pumps.
I was told to be on the low side in five gals you could be eight cubic inches low or high we set 4 low.
That's one thing now how much gas do you get in the tank do you put it on the center stand or leave it on the side stand , will the pump let you top off the tank , most pumps will splash gas all over the tank if you try to top off the tank , don't ask how I know.
My car is the same way it show better than it really gets , the better the mileage the larger percent you will be off to be on is to have had everything go right and that is never me.
The good thing is once you know the percent it's off you know just how far you can go.