There is one reason for Apple to ship new notebooks with 10.9.
Since 10.9 is supposed to get all around better battery life, shipping with 10.9 would let them claim the longer battery life numbers at launch, rather than just a silent update to the tech specs page after 10.9 is released.
I don't know if that's a reason that would really cause them to hold back the hardware update, but if they could wait an extra week and claim 10 hours battery at launch instead of 9 then it might be worth it.
That seems ideal. Makes sense because nobody wants to buy a computer and a week later have to upgrade to the new OS.