It's simply what each operator thinks it can feasibly run. Usually it errs on the conservative side, to ensure that few (if any) of the services in said timetable are cancelled.Forgive my ignorance but what exactly is a "strike timetable?"
Is it a guaranteed minimal service or is this still likely to have ad hoc cancellations at short notice "on the day"?
At the moment there is no guaranteed minimum service level during strikes. The government has said it intends to change this, but then again they also had this in their 2019 manifesto, and yet here we are in 2022 and there are still no concrete proposals!
If there are any cancellations (barring the usual train/infrastructure failures) I would expect them to be largely down to unavailability of signalling and/or maintenance staff.