I am though assuming a car with computerised fuel injection can build a reliable comparison measure of fuel consumed so that last months similar driving can be compared to this months similar driving.
So for example did changing the coils make such a big difference? I am not so sure yet. Normally i make many small journies. Idling for example definately eats into the petrol MPG consumption surprisingly quickly.
Afaik the car uses the number and duration of injector pulses as an approximate measure of fuel usage and the odometer for distance. The calculation will be approximate and will change with age and tyres.
I consider the display to be a toy and a distraction and never put much trust in them.
Petrol engines are horribly inefficient at idle because to idle they have to close a throttle plate and are literally choking the engine.