MUNI buses, except perhaps first thing in the morning, are essentially never on time. You can't go by any published schedule (which I assume is why Google Maps predictions are essentially random numbers)--you have to go by the NextBus real-time data.
I don't think the concept of "on time" even exists for MUNI. The route may have a defined start and stop time, but that's it. Throughout the day you can get estimates of the average time between two busses on the same route, and I have to assume that data is used to manufacture published schedules, but the schedules don't actually represent anything real.
You are correct. MUNI schedules are defined by bus frequency, not a timetable. Trains/Buses come every 10 min from 7-9am (for example), every 15 min. from 9-11am, etc.
So what data is Google Maps using for its predictions then? It's definitely not the same as nextmuni (at least last time I checked), and in fact seems much worse. I had always assumed there was a schedule and that Google was using it.
In some of the older bus stops there are approximate schedules. Before the stop near my apartment was removed it seemed to match the data on Google maps.
Google maps is much worse than nextmuni. It's one of the first things I was warned about when I moved here.
