I think it's more expensive to have Apple upgrade the RAM for you because you're paying for someone to put that RAM in the computer, as well as, taking the other RAM sticks out. And, considering that it's a Mac, it isn't as easy of a procedure.
Do you believe it's far more plausible that the machines are originally assembled in the various configurations? Your scenario might be true in the Apple Store, but certainly not on the original order.
Are RAM upgrades in-store the same price as the online store upgrade price?