Diff between diskutility and finder on free space
The reason is probably that you have local snapshots of time machine backups (http://support.apple.com/en-us/ht4878). The difference is that the Finder calculates the free space you can use (the backups would be deleted if necessary). They are stored at /.MobileBackups (at least that's what it has been in Mavericks iirc) if you want to have a look and verify how much space it is consuming.

Other possible causes:


some people claim that if you have disk encryption enabled those
values may differ 
maybe Disk Utility was opened before you removed
some files and didn't refresh?