I think a lot of Govt depts, didn't use the PPSN, or RSI number as a unique identifier in the past. They used a combination of DOB, address etc. But there has been a change to use the RSI as a unique number. So its quite common.
A lot of Govt systems like social welfare are linked to other depts now, so if you correct it in one place it should change everywhere. If you think about it lot of systems and records were not linked before the internet, and eGovt etc.