Confidence and competence don't always go hand in hand. Some people are very confident when they really shouldn't be, while others display little confidence despite being very knowledgeable.
Many years ago, when I taught in college, I became aware of a curious flip-flop factor. Some students (mostly young guys) who did quite poorly on exams were supremely confident that they had aced the test. On the other hand, other students (mostly women returning to college after raising a family) aced their exams yet were constantly worried about whether or not they did okay.
Perhaps this phenomenon is best explained by Darwin who said: "Ignorance more frequently begets confidence than does knowledge."
To sum it up, it's not unusual for a person to be a nit-wit and remain blissfully unaware of it. It's also not unusual for a person to be bright yet believe he's not smart enough or good enough. Let's look deeper into these phenomena:
Confidence Without Competence - Greek tragedies are noted for their portrayal of hubris. Many a character with arrogant pride and overbearing confidence brings about his own ruin. Pretentiousness prevents him from reflecting on what he doesn't know or on the possibility that what he does know may be dead wrong.
In the modern world, we have many examples of hubris in the lives of the "masters of the universe" who were "earning" stratospheric income in pre-collapse Wall Street. Did they know their own shortcomings? Did they seek to consider that they may be wrong? Or, did they simply display utmost confidence in a house of cards?
Highly confident people may remain ignorant of their faults for three major reasons:
Such thinking intensifies feelings of incompetency, possibly even leading to the "imposter syndrome". (You're faking it and if one just looked hard enough, he would see how incompetent you are.)
Competent people with little confidence remain that way for three major reasons: