Various model tape drives have published specifications for tape capacity in terms of raw and compressed data. According to the bpmedialist report, tapes can contain substantially less than the raw capacity and still have a status of "FULL".
If a status of "EOT" is returned by a write operation to bptm, Netbackup will mark the tape as FULL.
Note that the tape drive itself issues this status, and NetBackup is responding properly to an End of Tape condition.
This can be caused by bad spots on the tape cartridge, forcing the tape drive to skip sections of tape in order to successfully write a data block. This is analogous to a disk drive marking a sector bad and relocating the data to a spare sector.
This behavior occurs completely within the tape drive and is beneath the notice of the OS, and by inference, NetBackup.
Netbackup will write to a tape until the tape drive returns EOT from a write operation. This, and not any specific capacity, is what tells Netbackup to mark a tape as FULL in the bpmedialist output.
Tapes that are FULL cannot be used for further backups until the images on the tape expire.
To investigate why tapes are marked FULL (i.e. why the tape drive has returned EOT following a write) when the expected capacity has not been achieved, please contact the HW vendor of the tape drive.