It's not like copying files. Watch any render as it happens on the screen, it's anything but uniform. It's add dependent on the calculations needed for the segment of the scene being rendered. Shine a light on a flat surface and render, now add an object with a transparancy, another with a bump, another with a displacement, another with all three and a relfection, sepearate them and render now move them around and let them pass in front of one another render again, same scene, different rendering time, different calculations needed.