Because Apple supplies a fitting tool?
On the one hand, it's not very logical for someone to fall in between 2 sizes and to choose a size larger when it's an item that should fit quite snug.
On the other hand, this is not a very well thought through situation by Apple.
Well, that was exactly the advice I got from an Apple employee on Saturday when I went to the store to pick up my watch. Tight fit suggests I should order a size 8 band, but normal fit suggested a 9. The employee said literally, you should order a 9 because they are very elastic and will adjust to your wrist. Fortunately, they did not have any in stock on that size as it would have been a mistake. Now I am waiting for them to be available in store to go an try them, probably in a couple of weeks, before I buy one.