A psychologist is interested in how our visual perception can be fooled by optical illusions. Her subjects are students in Psychology 101 at her university. Most psychologists would agree that it’s safe to treat the students as an SRS of all people with normal vision. There is nothing special about being a student that changes visual perception.
A professor at the same university uses students in Women’s Studies 101 to examine attitudes toward violence against women and reproductive rights. Students as a group are younger than the adult population as a whole. Even among young people, students as a group come from more prosperous and better-educated homes. Even among students, this university isn’t typical of all campuses. Even on this campus, students in a women’s studies course may have opinions that are quite different from those of students who do not take Women’s Studies 101. The professor can’t reasonably act as if these students are a random sample from any population of interest other than students taking Women’s Studies 101 at this university during this term.