Finder too optimistic about free space?
Several things are happening here:


It sure looks like there are bugs in the space determination with mobile backups.
It's hard to tell in Finder since your user may not have permissions to read / calculate file sizes in many.
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.


Turn off local time machine
Set all local drives to be private in Spotlight
Boot into recovery mode and verify the boot volume / repair / back up if needed
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.