Missing Disk Space

I have 31 Gig in my raid 5 configuration.
When i right click on the whole drive it show 26 Gig use and 5 Gig of free space.
If I use the tree size utilty it show 20 Gig use and 5 Gig of free space. I went back to the drive and check the property of every folder it add up to only 20 Gig use (so the tree size utility was correct).
What is taking up the 6 Gig that NT report when I check the property of the drive, but does not add up if i check folder by folder?