After deleting a user account of about 800GB on a new Max OS X 10.10.5 Yosemite system with a 1TB SSD, I was left with only 400GB of free space instead of the expected 900GB.
A whole disk scan of the system by GrandPerspective reported that the scanned files (all of "/" ) were only about 35 GB in size, and that "Miscellaneous used space" was a few hundred GB, which should have been deleted/freed when I killed off the unwanted user account.
I ran Disk Utililty.app to "Erase Free Space" and it churned away creating temporary files to write to the "unused space". While that was running, GrandPerspective reported smaller "Miscellaneous used space" and the Finder reported around 700 GB of free space (up from just a few hundred). Then it jumped to larger "misc used" and smaller "free". Then it reported "running out of room on boot drive!" then it failed with a "cannot create temp file" error, but after I quit Disk Utility.app the finder reported 963GB free space, as did Disk Utility.app as did GrandPerspectives.
So the long and sort of it is that running "Erase Free Space" via Disk Utility.app freed up that free space.