The files aren't removed and the space made available until after Time Machines local snapshots are removed.
This is what I expect has happened, as you can see the amount of space being used for the Backups in your screenshot.
There are daily snapshots and one weeks snapshot, after that the files are removed. It is setup like this so that you can recover recent files you deleted (or modified) even if you don't have your Time Machine drive attached. Additionally if you run low on space the snapshots are automatically removed to free up the space.
If you must clear the local backups, disabling and reenabling Time Machine should do it.
You can read more about local snapshots here:
http://support.apple.com/kb/HT4878