I've had my ST auto for 3 months and have so far racked up 12500km. Whilst the usage has come down I still can't get any better than about 13.9l/100kms. Whilst I accept the fact my 2 inch lift, bigger tyres, plus bullbar and slim roof racks (small rola style) won't help consumption, I still think my car is using too much fuel.
You're right, and the first and most important reason is that it's too young.
Petrol engines might loosen up after 500-1000km, but diesels can take up to 15,000km before they settle (mine took about 13,000). Diesels also last for 500,000 or more km, compared to a petrol that might make 200,000.
I think Nissan put a fuel map in from factory that's fairly rich, so that the engine doesn't run too hot in its infancy. There's an easy cure for that - the ECU reset.
Easiest way is this: empty the tub & cabin, write down the trip meter. Pop the negative battery lead off, press the brake pedal, put the negative lead back on, then take the car for a gentle drive - no more than 2500rpm tops, try to keep the peak revs at about 2,000 - get it into top @ 80km/h and let the torque converter lock. Do this for about 10 mins then pull over, turn it all off, job done.
You can't hurt the vehicle like this. It's designed to reinitialise itself with a basic default set of values for injector timing and duration (as well as fuel rail pressures etc). Get it back to defaults then let it learn a really gentle fuel curve and it should be a bit lighter on the juice.
Just anecdotally, very 500rpm you use increases your fuel consumption by 2.5LPHK.