iCloud backup says 17g used in status bar but 8g when I click manage

Why does the status bar in iCloud say my backup is 17g and when I click manage it says the backup is 8g? According to the status bar I'm almost out of storage and when I go to manage it looks fine. So I have no idea what I might want to clean up to free up storage because I don't know which one is correct. This is the same on my MacBook Pro, iPhone and iPad.