I've had this issue for about 6 months. I assumed it would eventually sort itself out, but the problem persists. Several months ago I moved to iTunes backups for my iPad and iPhone. I delete my backups using System Preferences on OS X, but at some point noticed that about 500 MB of my iCloud space is being used for backups, but there are no backups listed on any of my devices that can be deleted (see screenshot). Any thoughts on how to proceed?