The extended wireless keyboard was just released alongside WWDC 2017 in June, so any possible update could be relatively minor.
The eject key could be replaced with a dedicated Siri key, for example, given the iMac hasn't had a built-in SuperDrive for CDs/DVDs since the Mid 2011 model. Siri is located in the same position above the delete key on the Touch Bar.
Apple will be releasing the Magic Keyboard with Numeric Keypad, Magic Mouse 2, and Magic Trackpad 2 in Space Gray exclusively for the iMac Pro as well, so the shipping estimate could be related to the new color being on the horizon.
Last year, a report claimed Apple was exploring a standalone keyboard with a Touch Bar and Touch ID for desktop computers like the iMac, Mac Pro, and Mac mini, but it's unclear if the product is still on the company's roadmap.
It's common for Apple products to face lengthy shipping estimates when they are due for an upcoming refresh, but it could also be nothing more than routine fluctuations in supply, especially given how niche this keyboard is.
(Thanks, Keir Ansell!)