Bro, it's a wrong piece of information you got. Doesn't matter what the manual claims.
iDSI is: Intelligent: Dual & Sequential Ignition. It's always active. In lower engine speeds, it fires plugs at different timings & at highes speeds both are fired simultaneously. That's the reason it kicks in after 4500 rpms. To save fuel, it can even virtually 'shut down' three cylenders at idle time or deceleration.
Bro, that happens coz City ownes don't drive it like iDSI deserves. I give you simple scenario: coz it lacks pick, people simply try to push it harder & press the paddel to the floor, that spoils the entire consumption record. Whereas in more accelerative cars, they'd press it gently & car gives response, that's why they get the same fuel average. If you drive City wih very wise gear shifts & lighter foot, it'd give you a lot better average, my old one used to give 18 in busy road of Lahore. When you push paddle to floor, it certainly comsumes more fuel coz of simultaneous dual ignition.
I don't agree with anyone on iDSI issue, it's a great engine, that almost equels direct injection diesel efficiency. It gives response like diesel engine, gives max torque at lower rpms (2800 I guess). It employes very high end & delicately balanced engine kinematics, it's got drive by wire torttle, it again burns what others waste, it's very clean in emission, it's technacially advance & innovative. And yet being 1.2L it still competes with 1.3L patrol cars.
And for those who buy an iDSI & think that they've somehow managed to buy a sports sedan in 8.5lac.... stop dreaming!