Unknown backups on main hard drive
Those might be Local Backups Feature of Time Machine.


  Local snapshots complement regular Time Machine backups that are stored on your external disk or Time Capsule by creating a local backup on your startup disk when your normal backup drive is not available. This provides you with a "safety net" for times when you might be away from your external backup disk or Time Capsule but accidentally delete a file. When your normal backup is available again, Time Machine copies the local snapshot contents from your startup disk to your normal backup drive.


Read the link above for detailed explanation.