When you put something into the trash, it is not deleted. It goes into the .Trash hidden folder on the same disk (and still takes up space). On the Mac, you see them in the Trash. If you execute the "Empty Trash" function in the Finder, the files are then deleted and the disk space returned.
You trashed the files, but you didn't empty the trash, so the files were still on the disk and in the Trash can, taking up space.
DiskUtility's wipe free space function does that. In your case, however, having no free space, it was unable to do anything.
Empty the trash.
I put the drive in a friend's PC, and there was tons of info I thought I'd erased in a .trash file (and several others I don't recall).
Technically when a Zero Erase or 7x occurs it should erase everything on the drive regardless.
However despite what software does, Flash type memory (including SSD's) is NOT erasing.
Apple has this to say:
With OS X Lion and an SSD drive, Secure Erase and Erasing Free Space are not available in Disk Utility. These options are not needed for an SSD drive because a standard erase makes it difficult to recover data from an SSD.
For more security, consider turning on FileVault 2 encryption when you start using the SSD drive.
Difficult, but not impossible.
I suspect the fact is that because Flash memory has a limited amount of writes, a 0x or 7x erase will wear it out sooner, so it's not enabled despite what the Disk Utility is telling you.
I guess as the march of Flash memory continues the only choice is to encrypt all files, then Apple gives the backdoor key to Filevault to law enforcement types and they use a device like this to download everything off your iPhone/iPad, Mac SSD, including the password.
Only secure method is the destroy the flash memory based storage
But like others have said, you didn't empty the trash before using the Erase Free space option, as there wasn't any free space.
Next time try using the Erase> Security option Zero or 7X, this will catch the whole device, then place your files on it.
however I suspect even then it's not going to do it.