Former Seattle (now Las Vegas) resident here. To make the buses run on time, the planners build in "time stops." These are locations where the bus should go through at specific times to not be too early for the rest of the schedule, and they also provide some buffer for buses to recover time lost due to bike loading, disabled passengers, and traffic congestion. If you're getting the bus after a time stop, you'll likely almost always get it on time. If your stop is before the time stop, bus arrival times will vary wildly.

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.