Snapshot Storage Size

Does anyone know where [VirtualMachines}.{SnapshotStorageSize] comes from?

It doesn't seem to come directly from size of the disk file reported in vSphere data stores?  Sometimes it matches exactly, and other times it is off significantly (orders of magnitude).  Is it calculated on more than just the snapshot disk size?

I'm trying to report on actual snapshot size.  The built in report uses TotalStorageSize.

Also - any thoughts on why [VIM_Snapshots].[Description] always comes across as NULL?   We use Description in every snap we take.