Solved DIsk space usage - different figures reported

If I right-click my C:\ drive in Explorer and select Properties, the disk usage is reported as 39.4 GB.

If I select all the top-level folders, right-click and select Properties the Size on Disk is reported as 29.0 GB.

If I show Hidden Folders/Files and the Protected OS files, then select everything, the Size on Disk is reported as 37.0 GB.

6 GB of this difference is down to hiberfil.sys

Can anyone clarify what accounts for the remaining differences?
39.4 - 37.0 = 2.4 GB
37.0 - 29.0 - 6.0 = 1 GB

Files Explorer cannot see. Or has access to see.
See: http://blogs.msdn.com/b/oldnewthing/archive/2004/12/28/336219.aspx

Thank you.

I tracked down some more info - there was over 6 GB of data in the System Volume Info folder. Not accessible by Explorer (for good reason) but I could see the contents in an Acronis True Image back-up.

Those folders/files look like System Restore Points - that would explain it as recently I did some big installs - a .NET update, MS Office, etc. In due course, those checkpoints will drop off the list and the space used will reduce.

So, total disk space used (right-click the C:\ drive and select Properties)
= size of folders /files in Explorer (including the hidden ones)
+ the System Volume Information folder size (configurable in System Properties > System Protection > Configure)