This is called the 'Dunning-Kruger Effect,' and while I'm not an expert on it, my understanding is that people experiencing it just do not have an awareness of what they don't know. They know enough of a thing to get by, and they have no cognition that there's more to know, and they use the power of rationalization to turn evidence of what they don't know into some other explanation. They're not wrong, someone else was, and so they were operating on faulty information, but this time they'll get it right.

I think Impostor Syndrome is a combination of things (again, these are just my understandings). One, just a general lack of personal security. There's a lot about the world that is designed to make us feel insecure so that we'll work hard to be able to buy things. There are literally whole industries built around it. Being insecure makes it very hard to feel like whatever knowledge you do possess is good enough because you're not doing good enough to get the thing.

Then you add an awareness that I think most people eventually have that you do not and cannot know everything, even for just one specific, non-personal subject. When that interacts with insecurity, it leads to devaluing the knowledge you do have because clearly there's so much more knowledge out there, so it gets really easy to say to yourself that you don't have the specific knowledge you need to do the thing in order to get the thing.

So in effect what you have is two people who are just kinda half-competently managing to get through life (which is the best any of us can really hope for), but one believes that their success is entirely of their own making because they lack external awareness, and the other believes they've gotten by entirely by chance because they have perhaps too much external awareness.

EDIT: I typed this too fast and there were a bunch of typos that I went back and fixed.