Difference between encrypted backup of an encrypted DB and an unencrypted backup of an encrypted DB
Does Rman Re-Encrypt TDE (Tablespace) Encrypted Data? (Doc ID 819167.1)

(above link requires an account associated with a valid support contract)


  When using column encryption, these columns as well as other data in
  the tables are encrypted again during the backup. If you are using
  tablespace encryption these are not further encrypted during backups.


...


  ... for compressed backup on an encrypted tablespace, RMAN must decrypt
  all the blocks, compress them, and encrypt them using RMAN encryption ...