Hi Brad and welcome to Apple Support Communities, babowa is correct in that if you make an order on the Apple Online Store (or via telephone) that custom orders can take considerably more time to ship.
The reason is simple.
"Stock-built" machines (those with the default configuration are kept ready to ship directly from Apple in the country they are bought in in almost every case. If you customise your order, give it a larger hard drive or faster processor for example, then your Mac most likely will have to be configured at the factory when it is manufactured.
This can easily add several days, or with some orders even several weeks to an order's shipping time.
For example when I ordered my Mac Pro, I could have ordered a base configuration and had my machine the next day, however I wanted a better graphics card, this added an extra 2 weeks to my product's shipping time.
When you are configuring the product (before you proceed to the payment screen), the "estimated shipping time" should auto update depending upon changes you make to the base configuration, as well as the extra time needed to make those changes to your configuration.
But I know from experience with the Apple Online Store as well as over the phone (technically it uses the same store) that you can always find out the estimated shipping time before you proceed to payment.
As a rule of thumb, any order that has "CTO" at the end means it is not a stock products and will always take longer to ship because it must be assembled first.
I hope this answers you question completely.
Regards,
Peter.