Finder and About this mac shows different free space sizes? Which one shows the correct value and how can I make room for ghost application allocated space?
TL;DR: Trust the Finder

Sometimes, about this Mac gives false info about how much space is taken up on your Mac. You can fix this by telling spotlight to reindex your hard drive. Go into system preferences, go to spotlight and under privacy, tell spotlight to not search Macintosh HD. Remove Macintosh HD from the list of exceptions, and spotlight will start reindexing your hard drive