There will be some exaggerated claims in mileage.
It always happens due to error in calculations and excitement of a new car.
E g. Some people will claim 17km/l at 120kph to 130kph.
While in reality the total drive would have speed of 100 to 105 for majority of the distance.
But due to error in calculations and unable to recall the exact speeds, people end up quoting the wrong speed.