It should all average out.
That is wishful thinking, really. The long-term average might be relatively consistent for similar parts of the day, but the variability between each single trip is likely to be statistically significant. I could drive the Line 6 route right now and do the same trip 60 minutes later after doubling back and easily get a different time by 1-2 minutes.
Without an official service alert I have seen many a time where two opposite direction Line 1 trains arrive and depart my station before even one Line 1 train in my direction arrives. Even with automatic train
control headway variance is not an uncommon thing. Consider that ~15% of Line 1 trains exceed the scheduled headway by 1.5 times. That translates to a delay of at least 75 sec for 2.5 min headways, or at least 2.5 min for 5 min headways. For a surface tram, even one with strong TSP by the likes of Paris T9, bunching is still a real possibility. Dwell times are often longer and less consistent than metros due to bottlenecked boarding and alighting inherent in tram design.
It's widely accepted that metros, especially semi-automated and automated ones, are less prone to bunching than less grade separated, less automated surface transit modes. Any delay on one tram is more likely to lead to more delays down the line than the same duration delay on an automated metro. In practice, manually operated transit is prone to cascading delays as human operators are imperfect and cannot consistently dampen delays like an automated system can (if programmed properly like Vancouver).
In theory, without adverse conditions, breakdowns or accidents, there should be no traffic jams on highways, and yet we all know phantom traffic jams happen. A driver following at a longer distance, who is smooth on the brakes can dampen the effects of a phantom traffic jam; but this effort may be for naught if the next driver following them decides to tailgate and slam on the brakes.
Any delay in the 1st train should prompt the 2nd train to slow down an equal amount to compensate, and then this theoretically should prompt the 3rd train to delay its departure from the terminus by that amount as well. But that often does not happen as terminus schedule adherence is often followed, barring exceptionally bad bunching or incidents. So hypothetically even if terminus schedule adherence is 15%, the bunching in the middle of the line could be 17% in reality.
https://www.youtube.com/watch?v=m74zazYPwkY
I believe you, but there's no reason this should be happening with a top frequency of 6 minutes.
I agree, there is no good reason this
should be happening. And yet it does. And yet phantom traffic jams happen. Theory will only take you so far.