But I still don't understand why some London buses on the same route still sometimes arrive almost one after another…….and then don't appear for ages
In Seattle this would happen due to really bad traffic, such as from baseball games drawing a huge crowd. The traffic would be in a pretty small time period, then suddenly disappear entirely. And this creates a knock-on effect where the delayed bus has more and more people now at each stop, more stops to make, and a longer delay in people maneuvering around each other to disembark.
Then the following bus, which starts out on time, has very few people to pick up, and ends up ahead of schedule because they usually can't stop anywhere to wait a bit. So people miss that bus because it arrives too early, and the following bus is delayed by having extra passengers again.
The other factor was that buses usually have another route before starting on their current one, and delays build up again. One route I'd ride featured buses feeding in from two different routes. Buses from one route were consistently late, and the buses from the other route were usually on time.
So generally it was poor planning and/or a failure to respond adequately to different conditions, even when those conditions were predictable weeks in advance
And a certain level of not caring how happy the customers are with the service probably contributes.