APA, MLA, and Chicago are only a few of the MANY citation formats that exist.
Studying chemistry is not the same as studying anthropology, so it should come as no surprise that chemistry students and faculty use a different style guide from that employed by their friends in anthropology. (In case you were curious, anthropologists tend to use Chicago style, while chemists frequently use the style guide published by the American Chemical Society or ACS.)
What are some of the other citation formats out there and who uses these?
You can see a complete list of subject areas and their corresponding style guides here.