Every time is a little different. I'm sure they have their reasons for whatever they do or do not do. I suspect that products without any remaining secrets already have all of their static content distributed across the CDN so there is no reason to pull the site down for any time. All that will happen is some minor logic will change based on a back end signal or even time and the buy buttons will appear.
That's how the Apple TV sale began, and it wasn't even right on the hour. It was like, 38 minutes after or something random.
And for everyone else, I would point to the second iPad Mini launch which was just like this. Not ever Apple product launches in some majorly synchronized manner. Sometimes... they just show up at some point. I am guessing it relates to expected demand and excitement. For as interesting as iPad Pro is, ultimately it's just a bigger faster iPad. Other than the pencil and dedicated keyboard port it's essentially a spec bump. My speculation is that the change in behavior we see is going to continue indefinitely as Apple begins to treat iPad like their laptops with minimal updates at a slower, less scheduled pace. I wouldn't even be surprised if iPad Air 3 is a spec bump without much mention at all. The numbers show us clearly that iPad is a declining business. I doubt it will go away, as the people who love their tablets *REALLY* love them, but, Apple is likely going to ramp down the focus, treat all 3 sizes as one line with serialized updates and move their innovation focus to other devices.
Just my guess (and also my supposition basis for the less organized launch this time.)