iCloud storage full but 5GB free?

I'm getting this message on my iPhone 4S:

This iPhone cannot be backed up because there is not enough iCloud storage available

I looks it up here http://support.apple.com/kb/TS3992:

This message is displayed when there is not enough storage space available in your iCloud Storage to complete your backup. Options are offered in the dialog to manage storage space. Alternatively, you can purchase more storage. If you received this message during a backup, or after recently being unable to complete other backups, you should examine and delete any incomplete backups.

When I go into iCloud storage settings, I see there is 5GB of 5GB space available. Any ideas what is going on?