Relation between RAM usage in Task Manager Processes and Performance

Is the amount of ram shown in the performance graph, 5.5 GB in my case :

the sum of the ram used by programs shown in processes :

Somehow I find it hard to believe that those 6MB will amount to 4GB and when I close firefox for example it seems the graph in performance falls down by much more than 830MB.

Is it just an impression, or are the values in Task Manager/Processes systematically lower than real usage ?

Edit.: Here is also the RAMMapp. Paging file is still disabled.