For space issues not explained by the above, there are a few things you can try, some of which may or may not apply:
Begin by restarting your computer as a first step. This sometimes resolves issues.
For Time Machine users on notebooks running Lion or later, space may being consumed by Time Machine local snapshots, which can be disabled by entering the following command in Terminal: sudo tmutil disablelocal.
Check to see if some of the space is being used by your sleepimage file.
Search with Finder to see if the space is being consumed by a very large file or several large files. Adjust the 50GB in the illustration to whatever size you deem appropriate.
Download and run OmniDiskSweeper (its free) It will provide a sorted list of your files/folders and show you where all your space is going. It includes system and hidden folderes
This above all will detail where your space is going. You have to consider your ~/Library folder it may not be included in the Other calculation and that folder which is normally hidden can hold a fair amount of space.
What is showing you this? If it's the Finder, it's possible that the difference is in the Library folder (normally hidden in the Finder). That stores a lot of stuff, like email downloads and iPhone backups.