incr backup Problem


I run an incr job everyday that copy data from two different drives on the same server. On one of those drive, the backup is taken in full (lun), on the other (physical drive on the serveur) incr. I don't understand why as the job is set to incr and on the two drive the archive bit is coherent. I can't take a full backup regularly because the data qty is too voluminous.

Thank you for your help!