There is some space used by the rather unaccessible (it needs tricks to access) System Volume Information folder. You can clear that by disabling (and preferably enabling again) System Restore. That operation makes it empty.

Then there's the 7% difference between the 2 definition of GB. That's .7 Gb difference to start with.

The final solution always (sometimes after a considerable amount of work and misunderstandings) comes from using programs like TREESIZE (free download) or CLDUW (free download also). They show you the size by folder and subfolder and subsubfolder and so on.
The size of all that folders (don't forget the root folder) combined should equal the used space (and so the free space) from the properties of the c:-drive (right click in My Computer) or even better chkdsk (chkdsk also shows space used by directories, file allocations, boot sector and other system areas).
Apart from file system errors (that can be repaired by chkdsk also) it always fits up to the last missing sector.

Kees