So a question is, why give a circuit board a name (I mean, besides just making it sound cooler)?
One reason could be if they planned to keep the same "Watch" name with no "2" or "3" etc suffix, and instead for the next release are going to simply say "the Apple Watch now with an S2 processor".
Conceivably it is modular and replaceable. I doubt it, but I agree with you that they aren't going to have an 'Apple Watch 2" or "Apple Watch Air." This is another reason they didn't call it the iWatch. They'll just stick with the Apple Watch name with no numbers.