I have a theory. Once you press the 'GO' button for the order you get the latest 'window' for dispatch and delivery. It does not get updated until close to the P4S for dispatched.
However, people who have ordered later than you can get an earlier set of windows due to the amount of people who have cancelled their orders. Therefore, even thou you have worst info than the person who ordered a day later, infect you will still be ahead them in the Apple build list. It is just that the info on your screen is not updated to reflect the real position in the process.
This also helps explain the under promise and over deliver comment. Also, when people have 'expedited' their order and seen an improvement on the order dates.
Info to back up this theory. During the weight for the iPhone 7+ I was following the thread and people 'updated' their delivery address to include their local county (in England) a lot of them got updated delivery info. It seems like that was a way to force the info on you page to update to the realtime info Apple have in their servers for the build process.
Again only my theory, if anybody has any others I'd love to hear them, because it make the 3/4 wk shipping pass quite quickly
(I have tried updating my order delivery address lat week, but nothing changed for me :-/ thou. Maybe I did it too early in the process and I am/was still in the 3/4 window )