It'll more than likely be the saved backups for Time Machine. Something that Lion introduced was saving its' own backups to your main hard drive, which would then be transferred to your backup disk when it's connected. These backups are automatically deleted if hard drive space gets too low, and I've noticed that Finder reports the disk space as if the backups are not there (as it's all a "background" thing), while iStat doesn't know any better and will report disk space as including these backup files.
Best way to tell is clicking on the Apple in the top left, then select "About This Mac", then "More Info", then "Storage". I wouldn't be surprised if there is about 60GB showing under "Backups".