Likely because they keep deleting stuff which is difficult in cold storage. They keep stuff for 30-days after you delete it.

https://www.backblaze.com/remote-backup-everything.html

> Backblaze will keep versions of a file that changes for up to 30 days. However, Backblaze is not designed as an additional storage system when you run out of space. Backblaze mirrors your drive. If you delete your data, it will be deleted from Backblaze after 30 days.