When NBU (or any backup application) removes old backups, those are recieved as deletes for files on the SteelStore CIFS/NFS share. The corresponding deduplicated data or references are deleted by SteelStore, and as appropriate the cloud storage slabs which hold the deduplicated data are reorganized or deleted according to what is removed and no longer relevent. It seems that in this case, the deletion of the data must be mostly references and not data.
If you feel this is not the expected behavior, then we may need to perform further investigation via a support ticket to NetApp support.