High School Assumptions

Why do people assume that since I went to a certain high school that I know everyone who ever attended there? Whenever the subject ever comes up which high school I went to, the person will say do you know "So and So" and 99 percent of the time I have no clue who they are talking about. They just assume that since there mother's, next door neigbor's, third child, who graduated 20 years before me and I went to high school at the same place that we were best friends. Test out my theory. Tell someone you don't know very well where you went and see what happens. I can almost guarantee they will ask. Humans are so predictable.