windows - Why RAM usage is much bigger than Virtual Memory usage

I am running some tests on a very powerful machine ( 24GB RAM, Window 2008 64bit )

I checked my task manager: Physical memory: 98% ( RAM is 24GB )

But the totally "Memory(Private working set)" usage of all my running processors is no more than 16GB

Where are those extra 8GB used for? Is it used by the OS IO buffer, or somewhere else?

Can anyone contribute some ideas?