That’s honestly a weird fact. I would consider the core 401 to be 409 to 404 but I guess there’s the 400, Allen rd, and a whole bunch of exits in between that relief the traffic. Consideeing the widest part like you said is just west of it, there’s also the insane bottleneck between the 427 and 409 that comes in between as well, generating a parking lot at all times of the day. I guess some 401 drivers use the 409-427-401 combo regardless so maybe thats why its not as busy as we might think either.