Thanks very much, Glyn, Jeff, and Tom. That was very clearly explained.
A related case, see the following top dump. The Postgres process is using 87g residential memory, which I thought was the physical memory consumed by a process that can't be shared with others. While, the free+cached is about 155gb. But, (87 + 155) is bigger than the total available 198g RAM. Does this mean some of the residential memory used by Postgres is actually shareable to others?