I don't want to justify the party line, but suggest the reasoning behind it.
I think with medical standards in things like eyesight, hearing etc. that tend to deteriorate over time, whoever makes these kinds of decisions draws an educated line in the sand as to where aircrew should be on entry, the reason being I would assume to ensure that a decent length of service could be expected from new recruits.
Once someone is in and trained, a good deal of money has been invested in him or her, so it changes the equation somewhat.. .This means that if people already working as RAAF aircrew have ears, eyes etc. that go below the limit, so to speak, their case will be reviewed on an individual basis and a decision made (not always favourable to the individual, either).
Makes sense, I guess.