I have an environment with a FAS2552 filer, it is configured with FCP against 2 clustered HyperV hosts, the cluster HyperV hosts have the block LUNs configured as CSV volumes, the problem is that between both hosts different occupation sizes are shown and In addition, the FAS2552 filer shows another size very different from what hyperV hosts see.
Space reclamation is enabled at the LUN level in FAS2552.
The virtual machines of the HyperV hosts have the disks provisioned in Think.
In the FAS2552 filer there is no volume level snapshot.
So you have to keep in mind which file system you are looking at, and if it is thin or thick provisioned too. A FlexVol can be thick or thin, and the same with a LUN (space reservation). There are 3 file systems, WAFL at aggregate, WAFL at FlexVol, and the host (NTFS?) file system. Windows only has visibility into the NTFS file system, but ONTAP only has visibility into the Aggregate and FlexVol WAFL file systems.
The LUN is just really a big file with special programming that allows a host connected over iSCSI or FCP to see it as a "disk". However, the space will never really line up, even with UNMAP enabled, because UNMAP won't reclaim all the blocks ever.
If it doesn't add up, honestly that's ok because it can't be even. Just make sure your volume has room for snapshots, and watch the space at the volume level if there is a high rate of change so that way you don't run out of space at the volume level.