Netvault backup size question

We are running backup 11.4. The server is backing up some NFS shares using NDMP. I noticed that sometime the reported backup size (in the job log) is much smaller than the actual NFS share, e.g. the reported backup size is 3.2 T while the actual size usage is 4.2T even if the job ran successfully.  Is this normal? Is the discrepancy due to the compression in Netvault?