There are a number of variable that'll determine how much fuel you can pump into your tank until the pump shuts off. The include (but are not limited to)
- Car's tilt (fuel tanks have all sorts of nooks and crannies)
- Pump's dispensing rate
- Pump's sensitivity to fuel foaming (determines shut off)
- Random slosh in tank during filling
And then there are numerous factors that can alter your actual fuel economy, including (but not limited to):
- Weather
- Traffic
- Ambient temperature
- Wind
- Terrain
- Your driving habits
That's why a single fill-up is unlikely to give you a true, average fuel economy. It could be an outlier on the bell-curve.