If you have TM on, that is almost certainly what is going on. When you delete files and empty the trash, they get moved to a hidden /.MobileBackups folder. If you run the command below in
Terminal you can see the size of the folder. Do not mess around in there and just exit after looking at the command output.
Code:
sudo du -d 1 -x -c -g /.MobileBackups
You can just ignore this and the OS will purge the data as the disk get closer to full.
Or, you can briefly turn TM off then back on and it will empty that hidden folder.