Strange 13.4 Gb Tape Capacity problem Dell Powervault 124T

Hi all,

Our Powervault 124T Autoloader backup only 13,4Gb on every tape, so 13,4Gb is the abolute limit.
We're using Symantec Backup exec to complete our backups.
Running a backup to disk job, there is no limit.
What could be wrong?

Donald