Nothing happens right on time. It appears right on time but it’s always slightly in advance or slightly late. Of course that depends on the time scale.
To be on time I aim for advance. It’s not even about allowing time for unexpectd events, it’s because there is no right on time.
Or rather, the only way to be right on time is to be in advance. So when the clock ticks, I’m here.
Being in advance has the nice side effects of (1) allowing you to plan one move ahead and (2) have ressources (time) to spare in case of unexpected events.