STAGEPROP ! Mystery behind the starting number !

I have worked on stageprop over the years on several issues but this is an interesting one that I had to wake up really early morning to help resolve.
Lots of clients have millions of records to STAGEPROP so some clients use a batch commits to avoid memory problems from loading the whole set. The usage of STGPKEY is very important to remember, if there is a STAGEPROP issue where the STAGELOG processed flag is manually changed states from unprocessed to processed and back. It is very important to understand that the starting number in STAGEPROP comes from the query below.

select stgpkey from staglog where stgrfnbr=0;