Most of the workings of Royal Mail's HR system are a mystery. I joined at my first attempt at applying, though the entire process took several months.
I can only guess at why the positions were withdrawn, it could be restructuring at the office, it could be someone leaving changed their mind! At our office most people have joined through applying for a job, in many cases our office isn't their closest to home but was the one advertising a vacancy. In the last few years or so we have had 3 people join directly after working as a temp/casual - of those, two have since left due to the way they were treated after joining, and the other has the distinction of being the only person I know to have been sacked for being useless!