Part of this is Speedometer calibration.

Fuel consumption is calculated in lab conditions, using an externally arbitrated distance and the fuel consumed is also measured under lab conditions.

Speedos will never be 100% accurate so manufacturers make sure that the error is conservative. Someone being charged with speeding when their speedo was exactly on the speed limit would be bad press. Most new cars are travelling at about 90-95km/h in 100km/h zones.

Petrol expands and contracts as it changes temperature. This is a good reason to only fill your tank on cold days, in the morning. (Although the cost difference is not worth considering, the difference in volume of fuel is measurable.

Finally, the fuel pump flow that is measured in your EFI system and the diameter of your tyres are also affected by temperature.

These three factors affect what your fuel gauge tells you. You may be travelling 100km and using 5.6l, but your ECU may be calculating and displaying a higher result.