On most public bus services the defined stops are so close together that there's only one plausible route they could take between any pair of stops; effectively the stops define the route.
But where that isn't the case, I've encountered variation between drivers. For instance, around a decade ago I often caught a bus out of Leeds bus station with its next stop on Boar Lane; about half the days, the bus would leave the bus station and turn left at the roundabout (along Eastgate) and half turn right (down St Peter's Street). Both ways worked; they varied in distance, traffic lights, and queues, so one wasn't clearly better than t'other.
For practical purposes (where passengers could board and alight the bus, and and what times) the overall ‘route’ of the buses was the same in both cases; it was a scheduled bus service, operating to a published timetable.