That makes sense.
That said, unless it has changed since my day, they normally phone when it is short notice before actually making the appointment. I remember getting a call from Chris Wildey on a Friday night asking if I could go to Eastleigh on Saturday afternoon. I told him I was at Edinburgh station ready to get the sleeper train at 11pm, he said he would use me as a last resort but would try others first (my date was closed, they were desperate).
I was appointed to a game at the end of last season, appointment at 11am, call at 1130 for that evening (rough timings). So they definitely can appoint before confirming.