Because you use iSCSI from an EQL which is a Block storage. The EQL doesnt know that someone from above have data deleted. If a OS deletets data it just marks it as deletet within the allocation table. Think about when formating a disk and NOT USING quick format. The OS  writes zeros and for the storage it looks like  valid data... but the OS will show it as free space later. For the EQL Volume it means 100% usage. Windows 2012R2 supports unmap aka space reclamation. It depends on your EQL FW and HIT Version and some other dependencies like Thinpro Volume on the EQL iirc. With that you you can tell the EQL that blocks can be deleted to get some logical space left. RegardsJoerg 
PS6000 v7.0.9 2 host Server 2012 R2 Hyper V Cluster Volume is 620GB Group manager reports 525GB in use Windows Disk managment and Failover cluster manager both report 349GB in use. Why the difference and where/how to I free up those 175GB?
SAN reports volume almost full. Windows reports volume only 50%