I just installed the newer version's beta or the Windows 8 CP on a 32gb flash drive and saw some 19.2gb available. That's slightly larger then a fresh install of 7. You wouldn't see the rest in the folder properties however since some drive space is automatically reserved for System Restore points and with the 32bit Windows you would need to take into account drive space reserved for the paging file used for virtual memory.
Restore points and swap space won't see any files there and remains hidden from view in most cases until you unhide all protected files and folders in the folder options view settings. The Recycle Bin also sees some space reserved as well since that acts like a temp folder when dragging things into it until emptied where you can still recover files from that.
All these combined take up the drive space you don't see while not being put in your face so to speak but tucked away neatly. These are all automatically maintained behind the scenes!