Why can't I change the active selected bone?

I am editing an armature object in Edit mode. I have the upper bone selected, and want to select the lower one.

To my knowledge, the last item selected should become the "active" selection. Once I shift-click the lower, it should become active and highlighted yellow. Instead, I get this after shift-clicking.

Both are selected but the upper is still active. I can’t seem to make the lower active. What am I doing wrong?