I also own an Altis 1.6 year 2015 driven 87K KM. The average in city is above 9.5km/l. On highway 12.3 if I drive at or above 120 km/h. however I have obtained 14.8 km/h recently when I drive 80 to 110 km/h on GT Road from DG Khan to Lahore. The more frequently you use brakes, less average you get. Also I usually keep 30 pounds tyre pressure and am using Michelin Energy tyres.
Your stated average is too low to be realistic.
But 15km/l is also too low. Reason is Toyota Pakistan is still using 3rd generation engines while 4th gen engines have much better fuel efficiency and can give better than 18km/l easily.