Memory leaks in VMWare workstation 9

Host: windows 7

Guest: Fedora 18

VMWare workstation 9.0.1

I gave the guest 2G memory.  However, after running a couple of days, it takes much more memory than 2GB.  The Recouse Monitor shows 2.2GB of working set, which looks normal.  However, after shutdown the guest, the OS instantly freed up 4.4GB memory.

Anyone knows why and what happened?  Is it a serious bug, or my misconfiguration?