Some people are very successful in their home towns, or their schools. They presumably gain confidence from this, and start to believe they are in fact better than everyone else. Then they move to a larger town, or they leave school and go to University. Mostly, if they are in fact talented, they will have the insight and curiosity to realise that talent is by definition relative, and that they may have to raise their game.
Some people however retain the big fish in a small pond mentality, and share their pedestrian insights on any topic under discussion. They’re the kind of people that might, for example, go on holiday to Australia sometime, and then spend the rest of their lives telling any passing Australian all about how poisonous Sydney funnel web spiders are. I suspect that in many contexts this air of intellectual authority may be a path to professional success, but it makes them very tiresome individuals.