That does not sound right.
If everything else is ok, your culprit might be one of your O2 sensors. I faced the same problem last year, and it turned out my O2 sensor was malfunctioning.
Luckily, we took it out and inserted it back in and it started working ok. The pick became better and the milage returned to 20-21 km/l (I drive with a very very light foot)