If a national geography class is categorized as a physical science, students are likely to be studying the actual physical terrain of whatever country the class covers. This means examining things like volcanoes, canyons, oceans, seas, and rivers as they exist throughout the country. In addition, this class might study the populations of various places in the country; however, it is unlikely that much time or attention will be given to the people themselves.
In a world geography class which is classified as a social science, students will still learn about the topographical components of a country; however, they are also likely to study the human, political, cultural, social, and economic aspects of the country. Learning how what happens in one part of the country affects what happens somewhere else in the country is part of understanding both the land and the people, and this is more in keeping with the social sciences.