Almost every road test reports fuel consumption below the official figure, and the gap is consistent rather than random. It exists because the two numbers are measuring different things on purpose.
The official number is a laboratory result
Certification consumption is measured on a rolling road following a defined speed trace, at a controlled temperature, with the car prepared to a specified condition.
Every element is fixed so that any laboratory testing the same car obtains the same answer. Repeatability, not realism, is the design goal of the procedure.
That makes the figure useful for comparing one car against another under identical conditions, and much less useful for predicting what a particular driver will see.
Real driving loads the engine differently
Test cycles use gentle acceleration and moderate speeds. Real journeys involve joining fast roads, overtaking, hills and traffic that force much higher loads.
Consumption rises steeply with speed because aerodynamic drag grows with the square of velocity, so sustained motorway cruising sits well outside the conditions the cycle emphasises.
Downsized turbocharged engines are hit hardest, since the cycle keeps them off boost while real driving does not, and off boost is where their advantage lies.
Everything the laboratory removes costs fuel
Test conditions exclude headwinds, rain, cold starts in winter, roof bars, a full load of passengers and luggage, and air conditioning running continuously.
Each of these is a genuine load. A cold engine in particular runs rich and with high friction until it reaches temperature, which can dominate a short journey.
Tyre pressure, tyre choice and even wheel size affect the result, and cars are frequently certified on the smallest wheel option rather than the one buyers actually order.
How a road test tries to be fair
A reviewer's figure is only meaningful if the route, weather, load and driving style are held reasonably constant between cars, so tests use a fixed loop wherever possible.
Brimming the tank at the same pump before and after the loop is more reliable than reading the trip computer, which tends to be optimistic by a small margin.
The honest comparison is between cars tested the same way. A single absolute figure carries much less information than the ranking a consistent method produces.
What the gap means for a buyer
Treating the official figure as a ceiling rather than an expectation is the practical approach, and the relative ordering of cars usually survives contact with real roads.
Plug-in hybrids are the exception worth watching, because their official figures assume a charged battery and diverge enormously depending on how the car is actually used.
Long-term owner data, where it is available in volume, tends to sit between the official number and a hard-driven road test result.