Jacks

I found a clear image of what the wiring for the input jacks should look like:

http://www.members.shaw.ca/house-of-jim/Images/Input%20jack%20wiring.pdf

I definitely would not have come up with this wiring scheme based on the layout and schematic I am using to build. It's strange to me to have everything wired to ground this way, but I guess if that's the way it's done who am I to argue. It sort of makes sense because the center switch is engaged when nothing is plugged in sending all hum to ground. On the other hand it seems like it would also send 'ground' from the unused jack through the hot tap of the jack that is being used. Maybe that resistor cancels that flow. I'm currently shrugging my shoulders.