About Bulb S being the brightest, I am not really sure.
Maybe it is closest to the batteries?
You know batteries contain a chemical that reacts to produce electricity, right?
Maybe when the electric current flow through the wires, it reaches Bulb S first, making it the brightest.
(Like the thing with how rich the oxygen is, depends on which organ it went first.
Lungs, followed by the heart, then the head and then, to the other parts of the body.)
Maybe, just maybe.