The simple answer to that question is that in 1992, not many people were collectors of first and last prefixes, especially last prefixes, as very few people knew when the notes for that year would have stopped being printed.
Even though there were almost 800,000 of the last prefix, most would have just been in general circulation and been destroyed by the time people started to be interested in collecting first and last prefixes.
From memory (and memory is always a bit questionable with an old timer like me), heightened interest in first and last prefixes began when the Reserve Bank began releasing that sort of information readily on the net around 2001.
Prior to that, the information was NOT freely available unless you were privy to employees or other private sources of information.
Cheers
Peter (notcb )