As I have found out those numbers aren't always true, particularly if it's a BTO.
Now, as for FedEx's number once Apple has actually given it to FedEx, my local FedEx guy tells me that often times Apple's packages will show up a day earlier.
In terms of leaving China, it rarely seems to leave earlier. There are numerous instances of this happening with BTO/CTO models. In my case, Apple said it was going to leave on a date, let's say the 1st of August (in reality it was around the 10th.) Two weeks later it still hadn't left China.
As for when it gets into FedEx's hands, that seems to be where a day or two can be made up.