I agree with everyone saying SS watches are probably sitting in a warehouse somewhere in the U.S. and SG is shipping from somewhere like China; hence the surge of "ready to ship" messages for the sport watches.
However, I don't think popularity has anything to do with it. I think we'll find out later this year that SS and Edition models were assembled in the U.S. Apple has been wanting to bring more manufacturing back to the U.S. The Mac Pro is assembled in the U.S. and what does the finish on that computer remind you of? (Hint: SS Space Black).