byte count decreases during backup

This has happened once and i thought its was just a one off thing but its happening again

I recently just configured backup to disk (GFS) to Nas using backup exec 2010 R3. I was running a full backup yesterday and by the time i left work, it was up to around 300GB. I came back this morning and its still backing up but this time, its around 190GB. No backup failure and no Cancellation. This happened the first time the backup exec was configured and i could not understand why and i though i was the one seeing things but its happening again.