The teaching unions prefer that the vacancy for a senior teaching post should be advertised but this sometimes conflicts with 'common sense'. I once attended an interview for a head of department job, in a secondary school, where the second in department had been doing the job for nearly a year, following his boss's heart attack. As soon as I arrived, I was told that the headteacher and governors were extremely satisfied with the performance of the acting head of department and that he was almost certain to get the job. I, and the other three external candidates, were simply there to make up the numbers, in order to meet the requirement that the post be advertised. That meant that four schools had been forced to employ supply staff (to cover for us candidates who were away from our classrooms) and the local authority, for the employing school, had been forced to pay four lots of travelling expenses and four hotel bills. (We'd all travelled hundreds of miles for the totally pointless 'interviews').
So it makes far more sense, if the governors are satisfied with the performance of the acting postholder, to simply give them the permanent job (rather than going through the time-wasting and costly procedure of inviting external applications).
Chris