*Obnoxious model engaged* You guys get
literally everything else first, cheaper, and in higher quantity, with better support, and priority in stock allocation. MacBook Pros over a week before us, iPhone 7 Pluses shipping way ahead of ours, and so on. So don't moan when someone else finally gets a bite of the cookie first! *Obnoxious mode disengaged*
Seriously though, we will probably find that they
do start arriving to some US customers on Monday, they are most likely being conservative with the estimate.
Other than that, it may be that in places like the UK the stock is already here in local distribution centres because the volume is much lower than in the states, whilst US orders will be shipping direct from China - with the addition risk of delay that entails, and hence a more conservative date.
Of course this is pure guesswork but I'd happily place a bet on some US orders arriving Monday.