I think both cases exist commonly. There are plenty of people entirely capable of A marks in most educational situations who don't get them because they don't

I think both cases exist commonly. There are plenty of people entirely capable of A marks in most educational situations who don't get them because they don't care and don't do the work. Something changes and they get interested. The jump from an indifferent C to a solid A often isn't that far. Somewhat related: There are more PhD's than you might expect that were also high school drop outs.