Disk used space discrepancy between SC8000 and FS8600

We have NAS server FS8600 and used space on 4 vols are less than 20TB. 

But in SC8000 array, NAS vols show about 35TB. Dell techs say this is because of white space on array, space needs to be reclaimed through SCSI Unmap on NAS server. So I turned it on and it's been 6 days after that and I expired last 5 day worth snapshots because they grow dramatically filling up vols in array. snapshots in NAS side hasn't changed. However, the discrepancy between SC8000 and FS8600 is still large. 

SAN:

SC8000

NAS Volumes: Vol 1,2,3,4

Configured Space: 60TB handed off to NAS

Active Space: 36TB

Snapshots: 2TB (Reduced from 5.5TB last night by manually forcing to expire)

Raid Overhead: 10TB

NAS

FS8600 FluidFS 5.0

NAS Pool: 60TB

Configured: 40TB

Used Space: 18TB

Snapshots: 250GB