Finder too optimistic about free space?

Several things are happening here:

1. It sure looks like there are bugs in the space determination with mobile backups.
2. It's hard to tell in Finder since your user may not have permissions to read / calculate file sizes in many.
3. Your spotlight database is suspect and might need rebuilding (where much of the data that System Information uses to make those nice colored bar graphs)

I would try three things to see if you can get a better view of your data.

1. Turn off local time machine
2. Set all local drives to be private in Spotlight
3. Boot into recovery mode and verify the boot volume / repair / back up if needed
4. Remove the volumes from Spotlight privacy and wait for the reindexing process to finish

You also might want to look into a tool like WhatSize which can measure folder and file sizes as the administrator (root user). Also, using df and diskutil list in the terminal will help you get a lower level look at actual file system data size summaries.