This could also indicate that the 15" is released first and the 13" may follow.
That's definitely a possibility, and one I'll admit didn't even enter my head.
Still, I can't imagine them updating the 15 inch first when the 13" is their best seller. I would say they'd upgrade 13" and 15" together, then maybe the 17" later, or update all at the same time, or update the 13" first then the others.
It's a possibility, I just think it's unlikely given the fact the 13" sells so damn much more than the rest! It'd make effective advertising difficult, almost impossible, as the people buying a 13" would see the ads for the "new updated MBP" and yet a lot can probably only afford the 13". Cue disappointment/confusion when it doesn't have the specs they have seen advertised.