Diff Linux agent backup is full backup?

We select using archived bit in backup option, and check no more option can be changed in linux and mac windows.The size of  differential backup of linux is same as full backup. Check backup history, it says "Differential - Back up changed files since the last full (using modified time)". How can we changed the backup of linux to using archived bit, so that even the backup is not 100%, it will back up diff data since last full backup?