cPanel backup retention behaviour works two in the manner the system.
1) Backups without Strictly Enforce Retention Option (Default)
Once cPanel & WHM completes a backup, the system removes the oldest backup. When the system-generated backup loses, the system will keep the failed backup, it will not remove the oldest backup, and will continue to hold the oldest backups. This way secures that the system keeps at least one successfully-completed backup. After the next backup successfully completes, the system will delete the oldest backups to return to the wanted number of backups.
2) Backups with Strictly Enforce Retention Option
Each Strictly enforce retention, regardless of backup success setting overrides the standard behavior for that backup type. If you select this option, the system will always retain only the desired number of backups with at least one complete backup.