I think it may be because of the varying surfaces of the road/ground that you run on. A treadmill is a consistent surface that gives a little. Out on the pavement, or grass, the surface is always changing forcing your muscles to compensate for balance a bit more. I think thats why the oradwork is a bit more challenging outside the treadmill.
Also for you younger guys, its far better to run on a track, than it is the street. They are made from a different material, have a little more give, and can be easier on your knees/joints.