Perhaps some hands on practice would help - try taking two cubes or spheres, subdivide them many times, switch to wireframe view, switch to orthographic side view, and then use Projected proportional editing (on a fairly small area) to move parts around on one. Then switch to the second object and use regular proportional editing. Switch into perspective front view and notice the differences between the two shapes. Then try the experiment again but looking from other angles. Hope that helps!