The original use of the terms went something like this: nerds are uninteresting (Potsie was a nerd - not smart, not obsessed with any particular topic, just dull), geeks are revolting (sideshow chicken head eaters were geeks). Obsessive interest in a topic can be both uninteresting and revolting, so a lot of people were both geeks and nerds by those definitions. These days, the terminology has shifted of course, but sometimes you still see the original uses popping out.