Yes, we are assuming that we have more information about specific FedEx flights than their tracking system *provides to us*. Certainly their internal tracking data is far more revealing than what fedex.com shows, but they don't share most of that with us. We've quite successfully inferred things like "in transit" scans in Shanghai that match flight times meaning the package was on that flight; a bunch of folks, including me, just had their MBPs arrive in Memphis on the flight that they were assumed to be on.
Also, various folks have already reported getting their shipments a day earlier than the "scheduled" delivery date. It's actually possible to get to about anywhere reasonably close to a major US city in 1 business day from when the flights depart Shanghai.
The airports outside of FlightAware's service area are of course a minor problem. However, for flights to/from their service area, they're generally pretty accurate. E.g., all the PVG-ANC and PVG-MEM flights that have been used so far showed up on FlightAware, and we matched people's tracking info to those flights. There are some things going via routes that would take flights that can't be tracked there, but the majority are not.
Basically, this is just a fun thing to do to pass the time while we wait for our new MBPs to arrive. Think of it as a game; I assumed my laptop was on the PVG-MEM flight when its second "in transit" timestamp exactly matched the PVG-MEM flight yesterday...and found out that assumption was correct about an hour ago.

If you don't care, then just stick with what fedex.com shows you. If you're more curious, then you can do fun things like track the plane your laptop is on in flight. See the mini-FAQ in my sig for more info, gleaned from looking at lots of flight data from FlightAware and elsewhere, as well as from lots of people posting their tracking info. Nobody's forcing you to try to have fun tracking your package in more detail...