The brochure figure is not invented — it is just measured in conditions you never get on the road. Steady speed, flat stretch, empty car, air conditioning off.
What actually moves the number
Short trips are the most expensive per kilometre, because the engine spends most of the journey cold. City driving with constant stopping and starting uses noticeably more than the same distance covered outside town. Tyre pressure, a roof rack left on all year and weight in the boot each add their own percentage.
How to find your own number
The only average that tells you anything is the one calculated from your fill-ups and your mileage. Log every fill-up with the amount, the price and the odometer reading, and after a few tanks you have a figure that describes your driving rather than a test cycle.
Once that number exists, everything else becomes measurable: what a month costs you, what a kilometre costs you, and whether the longer-but-faster route is actually worth it.