Why does the Memory from the Performance tab on the Windows Task Manager window not match the total listed on the Processes tab?

Why does the Memory from the Performance tab on the Windows Task Manager window not match the total listed on the Processes tab?
On this page, http://blogs.technet.com/b/supportingwindows/archive/2013/05/03/finally-a-windows-task-manager-performance-tab-blog.aspx , it states, "the Memory Usage graph (showed [sic] in Windows Vista/2008/7/2008R2) is the sum of all the process’s private working set."