I used to take the bus every day in Prague (Czech Republic) for about 6 years. It was almost always on schedule in both the morning and afternoon.
I don't know anywhere near enough to argue why in general. But not having drivers sell tickets, and route planning certainly help. For example, shorter routes are guaranteed to reduce variance. But I'm not sure what the downsides are.
It gets even more interesting when you add transport of the bus: the Seattle (King County) Metro also serves Vashon Island, so some of the buses travel on ferries with their associated delays and peculiarities.
It always stings to see the bus approach early such that I have to run for it, only to then have it wait for 5 minutes at the time stop 2 stops later.
I also do really like how Seattle has that dedicated busway twice a day. It's one of the avenues downtown right?
Edit: another thing that certainly helps is that most transit lines do not go through the typically congested streets (eg. no daytime public transit goes through the Magistrala in city center) and if they do they are separated (eg. Jecna).
That's easy! Even more with the proposed automatic system.
This information already is collected and available by text.
The city only needs to publish the ETA for the next bus on each stop, information that bus riders would also find extremely useful.
Busses get slowed down for any number of reasons. Bus schedules are basically fantasies, as any big city resident can tell you. There are too many things that can slow a bus down. Stop lights, accidents, securing wheelchairs, delivery vehicles blocking your lane, construction, police activity, congestion, the guy who gets on and discovers he doesn't have bus fare and wants to argue about it, and on and on and on.