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.
