Well, flaring too early will cause a hard landing (airspeed drops close to a stall), so will flaring to late (high vertical speed). Also at what point you idle the throttles is important. Just before you flare, your vertical speed will be about 700 fpm, or in other words 10-15 feet/sec, so timing the flare just 1 sec +/- wrong will put you 10-15 feet away from where you should be causing a non-smooth landing. Jan can give you much better advice than I can, but I bet he'll tell you a smooth landing is not something real pilots aim for because that can cause you to land too far into the runway (float) which might be dangerous, tail strike, cost extra fuel/time missing exit's, wear on brakes etc.