Discrepancy between instance PGA and private memory usage of DB processes reported by OS utilities

I am trying to understand Determining Oracle Memory Usage On AIX .

My original understanding was I would just add up, SGA + instance PGA at any point of time and That's it. But I was wrong!!

I saw on SGA + sum(pga_allocated_mem) from v$process is much lower than what the host is reporting as memory being used.

Then I did some digging and came across Doc ID 123754.1. That document described how we have to add SGA + sum(private memory) of all processes. When I used the "ps v" or svmon( -Ofiltercat=exclusive  -Ofiltertype=working) commands as explained that document, those numbers added up. SO my question is instance PGA reported by DB (