What you see is the historic fuel consumption of your car, being updated by the car computer since it was last reset to 0.
In simple words, when you press the left button on your speedometer, it changes the display between outside temp, current km/l consumption, Average( this is probably what you have seen in your car), driving range, average speed and elapsed time.
Talking about AVG , if you fill up your tank and you are in AVG mode, keeping the left button of the speedo pressed will reset the AVG to Zero. Now the car will calculate your fuel average for the whole tank, or till the time that you again reset it to zero.
For example, if you fill the car up and set the AVG to zero, you can see the average for the entire tank when its due for next fill-up. But during this trip the AVG will keep changing gradually, depending on your driving style. In the beginning it is easy to achieve average of 18-20 km/l, since the car calculates the average for the amount of fuel consumed, which is less in the beginning but as you consume more litres, the kilometers travelled per litre reach a more steady and dependable figure.
The car computer is very reliable. I have an Axio which gives around 14.5 km/l in Islamabad city driving. The average calculator works spot-on, every single time..