I have a 1st Gen Mac Book Pro Retina with 256G SSD, OSX 10.8.2
Basically, when I open Macintosh HD in finder, it tells me that I have 91.08 gigs available. Then, opening up Disk Utility and clicking on Macintosh HD, I'm told I have 46 gigs of free space. Just to clarify, RAID isn't set up and I should just have a regular hard drive.
MacBook Pro (Retina, Mid 2012), OS X Mountain Lion (10.8.2)
“You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed."