Types of memory: Why do private and shared not add up to real memory?

Question

Answers to previous questions define types of real memory used by a process roughly as follows:

Real memory: the amount of physical memory allocated to a process (eg the number of pages)
Real shared memory: the amount of physical memory shared between multiple processes
Real private memory: the amount of physical memory not shared with other processes

I would have expected that real = real_shared + real_private, but this doesn't seem to be the case. I've noticed that real memory can be more or less than shared + private, depending on the process.
Can someone please clarify the differences between these types of memory and why they don't add up?

MacOS Catalina 10.16.6

Ortomala Lokni · Answer

The answer to your question is given in Mac OS X Process Memory Statistics  by Mike Ash:

RSIZE: The total amount of physical RAM used by this process. (This is not equal to RPRVT + RSHRD because they measure address space, but this measures actual memory.)

Note :

RSIZE is Real memory
RSHRD is Real shared memory
RPRVT is Real private memory

You can find these abreviations by hovering the mouse on the Activity Monitor column headers.

Types of memory: Why do private and shared not add up to real memory?

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