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.