Yo Bro....i believe alot of lations share the same setiments as yours with lots of qns on the FC....like mine also has drop but mainly due to my route of travelling and % of highways vs cities area..so really gotta gauge the rational behind lor... Anyway, few factors that may cater to the reasons behind although some may / may not agree lah 
Air Con adjustment (How cold / hot you adjusted)
Tyres (Air inflation did you check freq)
Traffic Congestion (Traffics also need to consider the idling time, stopping period)
Foot how heavy (Heavier you are, more it will lim your petrol)
Speed constant (Maintain proper speed all the way to avoid unnec wastage of petrol)
Way of driving - Sudden brake / pickup, etc..(rem any sudden movement sure pump our ride heart heavy big time)
Route Area (City vs highway %)
Any more to add on Lations????

Hi,
I was advised and did the following
1. Air Con adjustment - 25 - 25 degrees
2. Air con pipe - wrap with black colour air con tubing
For tyre air pressure, I will pump around once a week. Am trying 250 Kpa (front) and 230 Kpa (back). The recommended pressure by Nissan is based on cold air. After tyre travel for a distance, it will get warm. Thus, I pump more so that when the tyre cool down and the air contract, I will be getting like 235-240.
Mine mileage is average 12.5
Rgds