POSIX path returns strange path?
The sandbox container directories contain symlinks to files the application is allowed read and write access to. For example ~/Library/Containers/com.apple.QuickTimePlayerX/Data/Downloads/ is a symlink to ~/Downloads/.

You'll get the sandboxed directory path when you open the file and QuickTime is already open and the "normal" path when you open the file and QuickTime is not already open yet.