I agree Sue. Can I assure our OP that there's many questions facing the informant when death registrations are being prepared. It is a duty that I have undertaken on a number of occasions.
In NSW (I am not sure about Vic) the informant provides the family history information to the Funeral Director at the same time as making the arrangements for the funeral, choosing flowers, music, pall bearers, venue, time of day (significant if there's elderly relatives/friends travelling long distances), and providing proof of ability to pay the costs of the funeral, choosing the coffin, the printing of the order of service, arranging a venue for refreshments after the service, and many other practical matters.
One of the series of questions posed is for the family history section of the registration of the death. The funeral director poses those questions during the course of all the other arrangements being made. So it is entirely possible that the person providing the answers may have misheard the question, or the person recording the answers may have misunderstood the reply. Of course, it is not just family that grieve, but close friends also grieve. At times of grieving, we can all mishear or become distracted by the numbers of questions, or perhaps the varying demands made by others when we ourselves need to be comforted.
These are some of the reasons that cause family historians to recognise that some of the information on death certificates is not as reliable as we would hope for.
Perhaps the lass who is living, but has been noted as deceased on the death registration, should consider contacting the Victoria BDM Principal Registrar, and seek to have the registration altered to remove the word 'deceased' and add her age at the time of the registration of the death. Likely there will be legal formalities to attend to.
Cheers, JM