Differential backups not deleting -- being marked for deletion only.

We are running Backup and Recovery 11.5 for Windows Server and have a GFS plan setup with the following retention settings:
Daily: 6 days
Weekly: 3 weeks
Monthly: 1 month

However, during cleanup, the Differential backups are not being deleted --- just marked for deletion. I thought differentials could be deleted because there should be no dependencies on the differential backup. Is this not the case?

Please see attached screenshot for reference.

Thanks!