If you attempt to compare the reported free disk space on your hard drive on Finder and Disk Map, you may find that the value being reported between the two applications differs.
The reason for this is because Finder displays the free space on your hard drive without accounting for temporary local snapshots (backups). Due to Apple’s sandboxing rules, Disk Map is only able to report the true free space available on your hard drive, which accounts for the space being taken up by the temporary local snapshots stored on your hard drive.
Therefore, it is likely that the free space reported by Disk Map will be less than what is reported by Finder.
You can read more about local snapshots at the following page: http://support.apple.com/kb/HT4878?viewlocale=en_US&locale=en_US