Airflow and performance are the big differences. Just because they are both made from oiled cotton, do not assume they are the same!
Also, the Focus ST specific filter has a reverse cone inside with the end of the filter open to this reverse cone for increased airflow. More surface area for more airflow. The K&N is closed off on the top and only flows through the perimeter of the filter.
Check out the link below to Green Filter's website for more details on the Green Filter.