Plug-in hybrids record official consumption figures far better than any conventional car, and observed figures from real fleets are frequently much worse. The gap is explained almost entirely by charging behaviour.
The official figure assumes a full battery at the start
Certification procedures test the vehicle beginning with a fully charged battery and weight the result to reflect an assumed share of electric running.
That assumption is a rule rather than an observation, and the resulting figure describes how the car performs when used as intended rather than how it is typically used.
A plug-in hybrid driven without charging behaves as a conventional hybrid carrying a large and heavy battery it never uses, which produces a substantially worse result.
Charging behaviour varies enormously between owners
An owner with a home charger and a commute shorter than the electric range can complete most journeys without the engine starting at all.
An owner without off-street parking faces a public charging session for a modest amount of energy, which is inconvenient enough that many simply stop doing it.
Company vehicles where fuel is reimbursed but domestic electricity is not create a direct financial reason to run on petrol, and observed fleet data reflects that.
The hardware penalty is real when unused
A plug-in battery is several times larger than that of a conventional hybrid, and it brings weight in cells, casing, cooling and structural reinforcement.
That mass has to be accelerated and carried on every journey regardless of whether the energy inside it is being used.
The vehicle also carries a full combustion powertrain, so it bears the weight and complexity of both systems simultaneously.
Where the design genuinely works
A daily pattern of short predictable trips with reliable overnight charging plus occasional long journeys is exactly the case the concept was designed for.
The electric range covers the routine driving while the engine removes any planning requirement for the occasional long trip, which suits households with one car.
Towing and cold weather are handled better than in a battery electric vehicle, since the engine provides both sustained power and cabin heat without consuming range.
Why measurement is now tightening
Regulators and fleet operators increasingly rely on data recorded by the vehicle itself, which reports actual electric running rather than an assumed share.
Certification methods have been revised to weight the result more conservatively, which has reduced the divergence between the official figure and observed use.
The practical consequence is that the vehicle's advantage now depends on demonstrated behaviour, which shifts the question from the specification to the owner's routine.