I probably could have made a career just on explaining disk usage to people. And while I did do a blog entry on the subject for AskCore, some of that information is aging out. The NTdebugging Blog just released part 3 of their series on NTFS Misreports Free Space. This includes some new information that covers Windows Server 2012 R2/Windows 8.1.
So if you are confused about how NTFS reports free space, you should give this new blog entry a read.