What I don't get is why is there a humcanceler pot on the back of series basees for EACH pickup?

They aren't humcancelling pots, they are hum-balance pots. The hum is adjusted to null for each pickup, because even if they have the exact number of turns, each pickup is slightly different and needs a different amount of influence from the humcancelling coil.

Now I can't say I don't know what I'm talking about! Thanks!
 
J o e y