I've been watching this thread for days, and haven't had anything to add, so haven't been posting.
Special thanks to TechDude16, and your UPS inside info sources - I've greatly appreciated your detailed and apparently right-on-target logistics info and reporting on UPS tracking and what it means. That sort of info is the best of what forums can be.
I find the 'hinterlands ship first' theory VERY intriguing, as it's the first vision of what orders are in what status that makes sense. I worked for Amazon.com for a long time, and worked pretty heavily with the logistics folks, though I'm not one myself at all. I hadn't previously gone through the thought process of 'you need to deliver 250,000 top-secret items on one morning, all to home addresses across the US - how do you go about it?'
With this question in mind, you instantly end up with a model that's trying to do a couple things - a) keep the packages secure as long as possible, b) try to spread things out so the bulge going through your logistics process is manageable, and c) ensure that the highest possible number make it on the specified date.
An obvious approach to this is sorting out ZIPs that are farthest from your major distribution centers, and pushing early loads to the closest facilities - this both allows you to spread out the load, and get shipments closer to your hardest-to-reach areas, increasing your chances of on-time delivery. It also avoids having giant piles of shipments sitting in Seattle, NY, Chicago, etc. for several days.
Then, push the urban-area shipments through at the last minute. These will be by definition larger batches, but they're all going to your major facilities, which can handle the load.
This also explains the Saturday launch - Saturday is the only day where UPS/FedEx have a significant 'surge' capacity. Their drivers, planes, and facilities are operating at a full 'normal' load M-F, and a light load on Saturday. By just extending the week, and using some or all of your M-F team on Saturday, they can handle a HUGE surge without too much difficulty.
Thanks to EddyP for the 'hinterlands first' idea - I think you're 100% on target. It makes perfect sense, and explains why my iPad is sitting in Guangzhou making me insane.