Actually, it's product ranging structure is similar to the phone strategy of recent years: available in white, space grey & gold which have modular integration with a wide variety of cases. Since there are no storage variants, it makes sense to offer differences in subtle materials or physical size.

Even with two subtly different chipsets for 38mm and 42mm that's hardly a new problem for Apple who have extensive custom chip experience across laptops, phones, iPads & iPods and not all of it in large volumes.