lower consumption is effected more by light foot rather than keeping under a certain rpm. i have tested this and can confirm, that at the same rpm and speed i can make the car (civic) 12km/l or 15km/l. using the same speed and rpm check how much average is the car meter showing you. if it's low raise your foot from the accelerator enough so the speed does not drop but the average goes up. try this a few times and it will improve your normal driving style in some time.
And yes, if you have a city giving you that much milage then either your average is lowered by driving in extreme rush areas rather than long stretches or you have a heavy foot. it is not normal for a city to give so bad a milage otherwise. figure out the reason.