Yea, I think I see what you are saying. New hardware release and new OS release don't always line up and this can sometimes affect what is "in the box". However, I disagree with some of your points as it has not been my experience nor has it been the experience of people I know.
I also don't think it matters much anyway. I believe there's already a backlog of BTO orders now let alone next week. So chances are if you order then you won't get an ML based system.
I just ordered my new 27" iMac BTO within the last hour. It is due to arrive mid to late next week. It should be here either just before or just after Mavericks. But I doubt it will come with it.
And I'm fine with that.