Restored a file from Time Machine when Backup Disk not available
I believe it is because Time Machine (in 10.7 and 10.7) keep local backups to augment the external drives.

He is an Apple support article.

"Local snapshots compliment regular Time Machine backups (that are stored on your external disk or Time Capsule), giving you a "safety net" for times when you might be away from your external backup disk or Time Capsule and accidentally delete a file."