It's probably because they have the other ones in stock to ship, but a custom one has to be manufactured. BTW, I think the Chip Beck spec seat is perfect. If I didn't find a great deal on a beautiful used K&H seat, the Beck spec would have been the seat I would have bought. Looking forward to seeing the pics.
I have ordered Corbins in the past, and I seem to remember 6 weeks is the standard delivery time, although they usually show up sooner.