Well a rib contusion might well lead to costochondral pain. That's the first comment i would have made. A contusion will cause a bruise that it pretty much going to be painful to direct touch and less likely than a costochonral issue to be painful with breathing.

In many cases supporting the rib cage against inhalation would reduce the pain of a costochondral problem. So to would some type of pressure supporting the joint more directly akin to something Mulligan would do.

It really was a pretty simple question and you had all the info to provide a simple answer. i.e. One was a direct trauma (assuming no joint involvement, and the other was a joint issue. Therefore one would have pain with movement of that joint and the other would most likely not.

Exams are funny things. For the most part examiners are not out to trick students, we just think they are. Often the simple explanation is the thing they are looking for. 8o