To me, it's more like the pizza place has five pizzas currently baking. You come in and order four pizzas, all with tons of different toppings including some toppings that aren't even available to regular customers. Someone else comes in just after you and orders one plain pizza. The owner pulls out one plain pizza and gives it to him so he can leave instead of making him wait twenty minutes while he ensures you're getting exactly what you ordered.
Except instead of four pizzas, it's fifteen thousand different book orders, and the other guy just wants a package of books redirected.
And like someone said, the books were all printed at the same time. So you have two sets of books. One set has to be unloaded, unpacked, and repacked into 15,000 different varieties of packages, along with a bunch of other goodies they may or may not have even received yet. The other set just has to be shipped out as-is. Shipping may very well have started around the same time, but it's obviously going to take a lot longer for individual packages to make their way out.
On another note, got my $68 package yesterday. It's awesome, and incredibly worth the wait.