no worries.. this is normal... this is because.. tape drive do compression before wirting the data to the tapes... so based on the data type you can have the more compression and write more data into the tape..
and netbackup shows the data size before the compression..
amount of data that you can write on the tapes depends on the compression ratio, you can see normal data files have more comperssion and they do write more data on single tape and reach full status
where as database files or media files have less compression and write little be less data on single tape and reach Full status.
looks you have the good compression ratio for the data that you are writing to tape that is the reason it is showing more data size..