Yes, shared memory is often not considered, because it can be used by many process - or it is counted several times.

A special type of shared memory is the SysV IPC (inter process communication), listed with

Code:

ipcs -p

E.g. for the "Shared Memory" the listed PIDs are for the "Creator" and the "Last attacher".

Even the normal fork/exec to create a new process produces shared memory.
Because initially they are identical, they are not actually copied but the new process reuses the memory. They are really copied when the new process modifies it. This is called COW (copy on write).