The odds say that about 1 of every 5 flights you take will be late. But odds are averages - some people have a run of worse luck and some a run of better luck. So you get the "I've flown XX segments in the last 6 months and haven't had a single late flight" and your unfortunate "I've had 4 straight nightmares". In theory, like flipping a coin, over enough time it averages out.

Jim