Quick answer: There's a lot more in RAM than the total of the "Memory(Private working set)" for all processes.
First, there's the shared working set for all processes. Now much of this is actually shared among processes, so it only counts toward total RAM usage once. But what task manager calls "Memory(Shared working set)" should really be called "potentially shareable". We can't tell from that display how much is actually being shared. All we can say is that the total RAM used there would be the largest of all of the processes. But actually there will be much more than that.
Then there are kernel space allocations, which show up nowhere on task manager's processes list. In the "Performance" tab though you will see indications of paged and nonpaged memory - those are the kernel pools. All of the nonpaged pool is in RAM all the time, and much of the paged pool will be also.
If you want to get a real look at "what's all my RAM being used for", look at the sysinternals tool "RAMmap". Note that on the "Use Counts" display the "active" column, and perhaps "modified", is the only thing that counts toward RAM usage.