You may not have heard, but we just announced the new iMac with Retina 5K display. Since your order has yet to ship, we automatically upgraded you to the new iMac with Retina 5K display.
Apple is probably doing this to most orders since Apple the new price of Retina 5K model is $1,999 and that was the price of the previous 27-inch iMac.
In addition to the new iMac lineup, Apple also launched new 15-inch MacBooks with Force Touch trackpads and faster flash storage. The company also started shipping a new lightning dock with audio line out for the iPhone 6 and iPhone 6 Plus.