Do you have NetApp Snapshot in use ?
Volume ( total size) = LUN ( total size ) + Snapshots ( size)
The above hold true if you are not using any thin-provisioning.
Now the real trick to understand is that the LUN ( total size) is the total size of the LUN. It doesn't matter if your client has 10MB or 100MB free space. It was provisioned from the storage and it's allocated from the volume. Hence, putting data inside a LUN will never contribute to the increase in the used size against a volume. ( Again to be CLEAR - This holds only true if everything is THICK-Provisioned - DEFAULT).