Backup exec duplicate to tape byte count

Hi,

I am new to backup exec 15 and currently on a trial.

I did a test job with backup to disk then duplicate to tape.

Everything seems to work fine, verify part succeeded, duplicate job succeeded, and I can restore data from tape and disk.

I am wondering why I see a different byte count for the backup to disk job and the duplicate to tape job.

For example, the backup to disk job has a byte count of 19.5 GB

And the duplicate to tape job has a byte count of 13.5 GB

Is that normal behavior and if so, what is causing this 6gb difference is quite major.

Regards