Is the Macintosh HD giving the most accurate reading? If so, try this:
First ensure that you have not deleted any data that you would need to use data recovery software to recover.
Open disk utility and select the Macintosh HD from the source list.
Click the erase tab and choose 'erase free space'. This will take several minutes.
In most cases this will bring the 'get info' output on the volume and the disk utility output back into sync.
Unfortunately, the "erase free space" option is grayed out. I restarted while pressing Command+R and used Disk Utility from the Recovery HD partition, but "erase free space" is still grayed out. I'm thinking I might need to erase the whole disk and restore from a recent Time Machine backup. Never done that, so I hope it will also reinstall all my software. I've already used the last of three 'installs' on my Microsoft Office license.
Well, here is the answer: Time Machine Local Snapshots
Beginning with Lion, for laptop Macs only, Time Machine takes 2 snapshots each hour. The first one is stored locally on your laptop, the second is stored on your Time Machine designated backup drive. The first snapshot is limited in scope to only files you create, modify or delete. In my case, I recently downloaded a lot of camera files to my laptop while traveling, so my local backup contains over 38 GB of data. That is the difference in the "space available" calculations done by the Finder and by Disk Utility.
More info here:
My first clue was noticing in Time Machine that there are two backups for each hour, one minute apart. The "local" snapshot backup is marked with a gray color tick in the column on the far right side of the Time Machine window. The "full" backup is marked with a pink color tick.