Backup process and leave files on local hdd

Hello i have 480gb for users, i have used space 400gb and that is 80% used space, when backup is enabled go to 100% used space, i have unchecked option: "Retain backups in the default backup directory." also backup is selected to FTP to remote storage. So my question is why cpanel don't remove user by user when create backup file and then upload it? cPanel backup all files and then upload them all together or how it's working?