I'm just assuming it's as simple as, they have X boxes of watch + band packed up (or expected to be packed up) and more than X boxes of just the band separate.
In the end, I won't be surprised if it all ships faster than expected. I had a thought that all the ranging and padding that is atypical of Apple represents an unwillingness to outright predict what people want entirely and gives them a chance to do some last minute rejiggering of their distribution across the models.