If you’re OSX disk usage is being reported incorrectly then you’ve probably got caught by a Time Machine feature introduced in Apple OSX 10.7. This typically happens where your free disk space erodes over time or when you delete some files and yet the disk still claims to have the same space usage as before.
This is a silently introduced feature where Time Machine can operate on on laptops (Macbook Air / Macbook Pro) when the TM volume isn’t available. The system essentially snapshots your changes for backing up later.
Read about it here http://arstechnica.com/apple/reviews/2011/07/mac-os-x-10-7.ars/18
If you want to disable the feature and reclaim your space go in to Terminal and enter:
sudo tmutil disablelocal
followed by return, and after entering you password and hitting return again, it should delete the local snapshots and free up the missing space.