df is not an accurate measure of how much space is taken by data. If I recall it is also calculating allocated space - that is, if your block size is 4k, and you have a file that is 1k, the entire 4k is allocated on disk. That extra 3k is reported in df, but NetBackup doesn't back up the white space, and so reports the sum of the actual data.
A difference of 7G from 258G doesn't strike me as out of the ordinary. If you want to be sure you can try summing the size of each file (which is rather tedious). You can also try seeing what quotas show, though I'd have to see exactly how quotas report data - they may suffer from the same issue. If you have the space, as a sanity check you can try restoring the data to another volume - you'll probably still see a difference, but (probably) not as big, due to fragmented files in the original copy.