Not any three overlapping sublayers can be regarded as a hub. What you think of as c includes five minutes of departure, five minutes of return and then departure. The overlap of these three sections is only a hub, which is less than the level of B, so there is no contradiction.

Although in lesson 17, the definition of center is the overlap of three sub-layers, in fact, it is mentioned many times in the following reply that the center is directional. Take the upward trend as an example, its center consists of three sub-layers from top to bottom, and the initial overlap of the three sub-layers from top to bottom does not constitute the center. It's just a center, and the level is definitely lower than the real center.

Read more replies about the original blog post. Many people have had this kind of question, and the pestering teachers or brothers have already answered it.