I've not had a lot to do with Thames Travel lately (they have retreated somewhat from mid Berkshire)
It did appear that they tended to base tenders etc on "highly efficient" schedules - or unrealistic running times and little in the way of recovery time. I'm not sure if or how far things have changed since they passed to Go-Ahead.
Ultimately, bus operators are expected to make best efforts to run to timetables in the light of likely traffic conditions (to put it another way, they can't timetable to expect the consequences of an accident or a car breaking down at just the wrong spot, but the timetable ought to allow for peak hours)
This isn't an exact science, as even peak hours will vary from one day to the next for no obvious reason, apart from more predictable delays depending on whether it's school or college term or both.
And from the passengers' perspective, while buses on time are what people want, people also don't want a timetable where the bus is timetabled to pass their stop at 8.15 on Monday mornings in school term, 8.18 other school days and 8.22 on school holidays...
http://bususers.org/complaints/how-to-make-a-complaint may be worth a look