chill man... no need post the same thing all over again...

first of all, do you understand what does L/100km means?

let me try to explain to you... the L/100km in your meter is actually forecast fuel consumption and not actual fuel consumption... 8L/100km meant that, 1 liter of fuel can go 12.5km... so the lower the L number, the better your fuel consumption...

so since it is a forecast FC, it wont be just be at one number, depending on how you drive the car... the more you accelerate, the higher the L number is... since it burn more fuel to go a said distance...

as for you getting 50L/100km when you in P and just reset, then drop when you move... it is because it calculates the fuel used VS distance traveled... when you start engine, it is already burning fuel... but since you're at P means you not moving at all, so distance traveled is 0km... let say you used 1liter and didnt move 0km, so it give you the max reading...
once you start moving, the distance traveled increased, so it forecast a better FC... lets say you traveled 12.5km and used 1 liter of fuel, so the reading goes to 8L/100km...

so its not an issue at all... the number in the meter doesn't control your FC but your right foot does...