Dssence
I would suggest the following:
1) the computer in the car makes changes to the timing based on many parameters. The reason the lower octane fuel gives poorer fuel consumption is that the car's knock sensor retards the timing when it hears knock due to the lower octane fuel. Once you get higher octane fuel in the car this should help fuel consumption from this perspective.
I've talked to a guy from toyota who said the ECU has a learning curve, is this true? for the first couple hundred km's. Which tells the engine how to inject fuel. Again could a lower octane fuel aside from any other theories be lowering the fuel consumption leading to less KM?. Something I forgot to state was that since I took the car off the dealer I used air conditioning extensively cause it's really hot here now. I thought on new generation cars AC wouldn't be an issue regarding fuel consumption.. but who knows.. just I didn't mention it earlier. Something is going round in my mind , could Toyota have sent these units to my country Argentina with a different ECU setting which is making more fuel consumption? When I talked to User support here, I couldn't believe they covered their asses saying Toyota ARgentina didn't publish Fuel consumption figures here can you actually believe that? so when I said the guys what should be the average consumption rate for city/highway they said we can't back support that as we're not authorized to specifiy those figures. 
2) in the owners manual there will be section that describes the dash buttons and the two that are used to reset the odometer, select the odometers, select the outside thermometer, the average fuel consumption display, the instantaneous fuel consumption display etc. Read this section, understand it and figure out what buttons to push to change the display value you see and how to reset those that are resettable.
I didn't know there was a real time fuel consumption monitor. Which of the two options is the instantaneous fuel consumption display, I need to check this. I know for sure there's one which shows the average. So the one you're telling me whenever I stop the car goes to 0. and then when I'm shifting gears varies accordingly to how I throttle the car and shift gears ?
Is G-on feature useful ? which tells you on the display when to shift gears for supposed fuel economy? or it's useless?
3) the average fuel consumption dash readout tends to show a little better than real life calculating the fuel consumption from the km. driven and the fuel added.
4) you should read and record the average fuel consumption, at refill time and then reset the average fuel consumption readout to zero. This will tell you what the computer thinks is going on. Record the odometer mileage since the last refill to full(keeping in mind the instructions on the inside of the gas cap regarding how full full is). Add fuel, and use the fuel amount added and the km. since last refill to measure the real world fuel consumption. You can then compare that to the computer readout to see the difference. Do this for 2-3 tanks of fuel( go down to 1/4 to 1/8 left in the tank before fill up).
Can you explain a bit more here. What I thought I would do was to go almost empty this fuel tank cause it's how I got the car from the dealer which I don't know which Octane fuel rating it has, so I would just use the car to empty the tank and then do a full refill with 98 RON OCTANE fuel from shell v-power nitro. Then I would reset the trip B to 0km. And then when I get where the second quarter tank begins with the needle I would read the trip B kilometers rolled.
5) Driving Style: fuel consumption from a driving style perspective will be a function of when you shift up and down and how much throttle you use. You can see the impact of throttle and shifting by setting the dash readout to instantaneous fuel consumption. It will read blank when stopped and change as you drive with how you use the throttle and shift, but go back to no value when ever you stop the car. The average readout will always show a number greater than zero until you reset it to zero. That is one way to tell the difference between the two read outs in addition to the fact that the average fuel consumption readout screen says avg and L/100 km while the instantaneous readout just shows L/100 km on the screen.
This is something I wanted to discuss.. believe it or not I come from a toyota celica 1979 liftback model and that was my jump to the FRS. 
so big jump. What I did feel was that the accelerator from my old celica I would rev the car like more specifically to the RPM's I wanted, and I have the impression " correct me if I'm wrong" that when I accelerate on the FRS the revs go to the sky I cannot rev accordingly to the RPMS specifically like I could do with my old car. Maybe I sound stupid, maybe it's a matter of getting accostumed to a new car like this one. Maybe this sudden revs is making the fuel consumption excesive on my foot.
6) you will see some very high instantaneous fuel consumption readings as you accelerate but lower values when cruising. The shorter the duration of higher values and the more time at low values will result in better fuel consumption. If you are seeing real world fuel consumption worse than the worst at the fuely website for this car then you may have a problem. If you are in the range shown there then you probably just need to work on your driving technique to improve to the better end.
Certainly that was what I was referring to on the previous statement. I think I'm not adjusting properly the REVS on my foot , when pressing the accelerator. I don't know if that's cause the car has an accelerator sensitivity issue.. or it's my foot problem. Again Average fuel consumption per litre for 100km what should be a figure here. If this doesn't improve could the ECU be programmed badly for fuel optimization?
Try this for a few tanks of fuel and see what you learn about your car and fuel consumption.
Just a suggestion.
Please note that if you have the dash readout set to avg fuel consumption, your readout can show a higher value because the first thing you do after fill up is accelerate away which uses way more fuel (particularly if you are heavy on the throttle) than cruising on the highway. Over the time to next refill, the average should even out depending on the mix of city and highway driving. Remember the throttle is not an on/off switch! You are allowed to use less than full throttle to accelerate!!
Additional Note : whenever I start my car.. I see the car increases revs for a couple of secs.. and then stabilizes is that normal?
Thanks so much ! I really apreciatte all your help and time
Cheers