It's not that people are 'dumb' or 'smart'. It's just that some people have an easier time with some topics than others. I have seen people who really struggeled with the basic concepts after three courses (not my courses ;)). Others understood it far quicker and with much less work.
I'm not saying that these people were dumb. They usually excelled in other areas, just like I am seriously unable to keep a plant alive for more than a few months (believe me, I tried!).
As I said, to some people some topics are an exercise in frustration. It's not that hey can't learn it. It would just be really painful and boring to them.