Generally as you stated on time is + or - 15min of the scheduled gate departure, and gate arrival time. If you use flightaware or flightradar24 they often use the estimated takeoff time, which they usually have at 10 min after gate departure. It looks like the OP is defining ontime as pushing back from the gate at the scheduled gate departure time. He is not going by the 15 minute leeway.

I do agree with the OP though. I fly quite often out of YHZ on AC603, the 5:25am departure for YYZ. I've never seen it pushback at 5:25. It's usually closer to 5:35 or 5:40 before we are off the gate. However depending on taxi times, and the winds, these late pushback times often don't effect the arrival times too much.