Wait! They tested the coffee served by a café and not brewed themselves? How could they make sure that the beans are the same? How could they vary the beans sor

Wait! They tested the coffee served by a café and not brewed themselves? How could they make sure that the beans are the same? How could they vary the beans sorts and the roasting/grinding processes but keep the brewing methods to make sure the difference is the result of the different brewing processes and not of other factors?