I dont know. They could forgo the preamp function of the dac3 and maybe some of the inputs. It may not be that big of a difference.
The inside of the dac3 is basically 3 boards and 2 blue power supplies (elpac? I dont know the name).
The dac chip sounds like it is different, but the output stages may not be that radically different. Just speculation, obviously.
I have some photos of the dac3 somewhere, but I forget where. I cant seem to find a pic of the backside of the ultradock, so at this point there's no way to know.