This may also be due to a few other factors, for example:
- Encryption of data (either in Oracle or during backup) - encrypted data does not de-duplicate well as every time a small piece of data changes the entire chunk of encrypted data appears to change (due to the encryption algorithm). When using encryption you should normally expect low de-duplication ratio but reasonable local compression ratio
- Compression of data (again either in Oracle or during backup) - as with encrypted data even if a small piece of data within the database changes the entire chunk of data which is compressed together appears to change (due to the compression algorithm). As a result compressed data normally gives poor de-duplication AND compression ratios
- Use of binary data embedded within tables - binary data such as image/media files tends to be pre-compressed by its codec so is essentially compressed data and all of the points above apply
- Backing up archive logs - archive logs contain transactional data which tends to be very unique from backup to backup - as a result de-duplication ratio for archive log backups is generally very poor however these should still compress (if not compressed during the backup)
- Very high change rate within your databases - if the majority of the data changes between backups then most of each backup will appear to be 'new' data which the DDR has not seen before. As a result it does not de-duplicate well (however should still compress if not compressed during the backup)
Hopefully that helps a little.