It's called a credit hold, and it happens all the time. This is done as an agreement between the merchant and the card's issuer to ensure that the money will be available come time to ship the product. Otherwise, you might order a custom watch, then max your card out before the watch ships. Then, when the merchant goes to charge the card, it will get declined and the merchant is stuck with the product (not that Apple is going to be "stuck" with the watch, but you get the point).
Whenever I pump gus, as soon as I swipe my card a $100 hold is generated in [state] to insure that there are funds available for the merchant to collect for the product I am dumping into my car. This is done even if I only pump $1 worth of gas. Basically, it's a "good faith credit guarantee in the amount of x".
There is nothing uncommon or strange about it. It's a regular event with credit cards. Since holds disappear once the funds have moved, most people don't even notice.
So, yeah. If you want to order a product 6 weeks in advance, Apple will place a credit hold on your card to guarantee they are going to get paid when they ship your watch. Or you can just wait.