The solver drives me batty. I rarely see reason for what it chooses as redundancies other than a myopic view of previous actions to the redundancy and not the subsequent ones from the time "it" decides a specific constraint should exist, even if they are still weak.
Don't give too much mind to this shortcoming. In general, the training is only trying to show you how to handle conflicts. In your case, it is overlooking the fact that it can extend the horizontal line and internally it is thinking moving only one end of the arc that is vertically constrained to the other. The solver is simply to quick to find fault with your technique when it isn't very good at actually solving or finding all the conflicts.
What better way to learn than to learn that you might just be smarter than the software. Valuable life lesson 