9 average is indeed a bit low for within city average, but do NOT expect to get 15km/L in city driving. Vitz never gives this average and anyone claiming as such is lying / calculating incorrectly.
I know you are saying you have tried all the things, so it’s still a bit confusing why it’s giving you max 9/10. Have you reset the average mileage meter yet?
In stop and go city traffic the max you can expect is up to 12 km/L.
With heavy foot and very congested traffic 10-11 is fine.
Only on empty roads where you don’t brake at all and drive steadily at 60 speed 15 average is possible.