Fragmentation of system files. He can't compress if at the end of the section you have is a piece of swap file and the hibernation file for example, VSS or vault.
To get rid of this -
Disable the pagefile, disable hibernation, delete all shadow copies, reboot and make defragmentatie. After defragmentation squeeze and turn it back.
Well, as you already properly noticed to install two systems on the disk this volume does not make sense, the more it seems you have SSD. Then it's all pointless.