If you have windows installed on it, perhaps windows is saving restore points on the free space? (ex, you move files, free space, windows uses it for the restore points.) I've had a similar issue and it was due to windows saving restore points on the free space I had.
--- You can manage this by looking for "system restore"/"create a restore point" and then click "configure", there's options to see how much space it can use for restore points. Also you can delete old restore points using disk cleanup, and then do "clean up system files" and it will delete all restore points but the latest one. See if that works? Hope this can help
--- More in depth info:
- System restore space
- Deleting old restore points