For me to get 8km/l in the city I would literally need to be stuck in traffic at least an hour every drive and practically floor the gas pedal every chance I can get, sounds a little unrealistic. The 14 km/l in the city would be perfect conditions of clear roads without many stops (something like driving only during odd times), which is also somewhat uncommon.
The point was to give a realistic expectation of a day to day fuel consumption, rather than quoting the absolute max/min achievable, for the people reading and potentially thinking of making the shift to CNG or not.