What is the logic to the order of Shimano Components?
For road components Shimano has the following quality ranking, from top to bottom and from (very) expensive to cheap: Dura Ace Di2, Dura Ace mech., Ultegra Di2, Ultegra mech., 105, Tiagra, Sora and Claris.

What trickles down from year to year are the innovations. If some innovation appears with the pro-groupset Dura Ace it will appear in the following or second year in the Ultegra group and so on downwards while the the Dura Ace components have some further improvement.