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