i think its due to the "anchor" position of the pulley.
with a free wight there is no "anchor" so the delt can be effectively isolated to move the weight whereas with a cable i always feel my forearm kicking in a bit to counteract the anchor point of the pully and the plane of motion of my arm... not sure that makes sense. but its what it feels like. my theory lol. i obviously don't have a degree in physics and etc.
-JS
I agree.