System storage not being correctly represented in 'About This Mac'
Try reindexing spotlight:


Go to System Preferences>Spotlight
Go to the Privacy tab, and add Macintosh HD the list.
Restart/Reboot your Mac.
Go back to System Preferences>Spotlight>Privacy and remove Macintosh HD from the list.


WARNING: This will consume large amounts of power (at first you can expect about 1% of power loss every 15 seconds or so, but it will slow considerably). It will also slow you computer considerably, using up almost all of one of your CPU cores, and about 2 GB ram. This is from my experience on a 1 TB hard drive, so it may be less or more intense for you.

P.S. Plug in your charger before doing this.