UPS offers many services other than simply picking up packages from a company's warehouse/office, and they don't all mean that something has been "shipped" officially. You ever see those UPS commercials where they talk about solving problems, logistics, etc...? That's what this is. UPS has vast warehouses where they store inventory for customers, and those packages are not considered "shipped" just because UPS has them.
It's very likely that these units are arriving from China directly into UPS warehouses and then getting processed for delivery directly to consumers. It doesn't make sense for Apple to maintain a huge set of warehouses for temporary storage of stuff like this, nor to have these big shipments going to an Apple warehouse somewhere only for them to turn around and have UPS pick them all up again to ship to customers.
All of this would be hidden from customers, as there's just no need to over-complicate the information that customers receive. Normally it wouldn't be a problem, but for a highly expected product like this, everyone is obsessively checking status and paying more attention than normal, so the nuances in each status get more scrutiny.
UPS also has to maintain a certain level of service, and part of that is reporting. They would have to report on how often they hit or miss their guaranteed delivery times (not Apple's estimate but their own standard of service for each type of shipping they offer {e.g. "overnight"}), and something shipped "overnight" needs to be delivered within 24 hours (or whatever) in order to maintain those numbers. They will not mark an overnight delivery as "shipped" until they actually want to start that 24 hour clock, and since they're not allowed to deliver until Friday, they give them a status other than "shipped" until they can comfortably assume the delivery can be made. This would only apply to logistics customers, not a typical retail customer (before anyone starts making accusations that they could just hold everyone's packages all the time to always make their numbers look good). All of this would apply to many other carriers as well (Fedex, etc...)
Now that we are within the 24-hour delivery window for "overnight", I would expect to see those statuses changing more to shipped. Of course, deliveries can also be made after Friday, but anyone seeing "Preparing to ship" can be confident that their wait time for delivery will soon come to an end.
Watch 42mm Stainless Steel Case with Black Sport Band.