it backs up files that haven't been altered since the last back up
A backup (other than Full) should only backup files with the archive bit set, whether that's Incremental or Differential*.
If you set it to do a Full backup, then it will do so, backing up all files - irrespective of archive bit setting - and clearing the archive bit on all files.
If it is set to do an incremental, then it should only backup those files with the archive bit set, and then clear the archive bit. It will not backup previously backed up files (unless they have changed, been moved or you manually set the archive bit). This makes backups shorter, but restores longer (last full backup plus all subsequent incrementals).
A differential backup will backup all files with the archive bit set but not clear the archive bit, so it will always back up all changed files (archive bit set) since the last Full backup. This makes backups longer, but restores shorter (last full backup plus just the last differential).
Sounds like you are configuring Full and / or Differential backups. Or else the backup program doesn't utilise the archive bit correctly (unlikely), or the clearing of the archive bit has been overridden in the program settings.
*
Allegedly, the backup types were going to be called Full, Incremental and Excremental, but wiser counsel prevailed...
SD