Why does the df command not show correct values for nfs mounted volumes when served by NAS such as NetApp/EMC filer?

Issue

• df command is reporting more usage than what's visible from the share path:

  # df -h /test
  Filesystem                 Size  Used Avail Use% Mounted on
  nfs-server:/share  100G  100G  708M 100% /test
  
  #  du -shx /test
  46G     /test
  

• several files are removed, but space is not reclaimed as per df -h output.

• the actual usage doesn't match up with du.