Yeah I think Qantas has the intent to hire every cadet, but they have also given themselves a safety blanket just incase a crap cadet somehow got through their application process (no process is perfect).
Also if you aren't guarenteed a job, then you would work harder then if you knew you were assured a job (competition).
That figure of $75K is a guideline, as ideally a perfect cadet will do X amount of flying hours multiplied by hire rate comes to $75K.
However if you are struggling in certain parts you will have to redo certain lessons. This can add that extra $30K - $40K depending on the ability of the cadet.