I know your TOC is highly reliant on overtime anyway, but I am surprised giving the reduced services and turns required!
There is a lot to do with how the rosters work and how inefficient and rigid they are. Training and booked leave are still ongoing so you need that to be covered. Depot sickness is up and up at other depots so you also need to cross cover work. Our Drivers are still being pulled off for assessments too.
The diagrams are all STP/VSTP but they don't reflect the normal establishment levels or master rostered diagrams, so you get a situation where SP cover falls outside the normal movement and you have an inbalance with early lates. We have composite diagrams that need to get covered and are usually because one Drivers is being replaced by two or more.
All things being relative, although the service has been reduced. So has staff availability.