Weird free spaces estimation
This information is calculated based on the Spotlight index. If your index is incorrect, this information can be incorrect too. Try reindexing your disk:

sudo mdutil -E /


sudo elevates the command to root to allow full control on the system. mdutil is the tool for controlling Spotlight. -E is the reindex option and / specifies the path to reindex. For more information on mdutil, see the man page.