This probably an old rort that has been going on for many decades and a" left over" from when QF was government owned. There was probably never a system to check the claims and nobody has bothered to create one or, they have just looked the other way.
Present any possibility to defraud and there will be those who will find a way to take advantage of it.
A similar thing happened just a few years ago at Dragon Air in Hong Kong.
I was told that pilots were presenting false receipts for inflated rental rates to the company for reimbursement over many years . When Cathay took over, a quick check of the books exposed the fraud.
Certainly ,retribution if not restitution was swift and caused a few Dragon Air pilots to return to Australia to look for work.
I believe that it went on for so long because some management types were among those doing the defrauding.
In any case, a crime is a crime and by making a false claim, the crime has been committed.
They had better just not look too far back. It would be an interesting exercise from the stories that I have been told over the years.
It could end up being a new income stream for QF!