People are not eating. To desert a place (in this case) is to leave it empty of people. Desert is connected with the the state of being empty. Everyone is trying to cram (get information quickly in a way they probably are not enjoying nor retaining the information) for their final exams.

here deserted mean absence of crowd. thus The snack bar is not always this deserted= The snack bar is not always this less crowded. simple :)