With operators changing timetables all the time, PCB (and Go Cornwall for that matter) do not like adding additional buses to up the PVR during peak periods. They do however change the timetables on routes effected by roadwords by upping the PVR and extending the travel time to counter the traffic. but it often takes them weeks and months to get round to this, by which time the roadworkds are nearlly finished. Then to save miliage, a few weeks later, the timetables change again so vehicles can be moved around to other problem areas.
SC in Plymouth at least, especially on the 1, will slot in buses at Royal Parade to form a RT departure if the inward workings are delayed. The late arrival then just picks up the following working. When circuits are running to time again, buses are then pulled from the circuit. Yeah it can mean buses are late and journy times are extended due to traffic, but the frequency remains almost the same. With PCB if a bus is delayed on an inward working, then a circuit is missed with a gap in the timetable.
Operators like NXWM & SC are well rehearsed in adding and removing vehicles from services throutgout the day to maintain frequency (while admittadly, not running to a timetable), about time Go-Ahead took note.