Sorry Romiii for the late reply..
No, my car avg is fine, almost 15 Km/L.
I asked Toyota guys, they said there is not any problem in the car. It happens in the Vitz that the car releases itself after some time. Still, I am not satisfied.
If you use high octane, the problem becomes lesser. Moreover, when the car is decelerating, if you put your gear to Neutral, everything becomes perfect. From the above observations, it seems to me that in low speeds, car remains in the gear (low gears) and it behaves in the same way as any manual car in low gears behave when neither accelerator nor clutch is applied.
Please share your observations too..