This does not surprise me. Way back when I interviewed for my first academic job at a state university in the South, an academic dean told me that students who came to the university after graduating from a “junior” college were more likely to graduate than students who first arrived on campus right out of high school. His explanation was that those “transfer” students on average were more highly focused and motivated to earn their bachelor’s degree. So there’s a “selection effect”: students who went the community/junior college route AND SUCCEEDED were hard working and generally had a good idea what they wanted to study at the major university. It’s not that they were smarter, but they were goal oriented.