Maybe in an environment where most people show overconfidence about their abilities, people expect overconfidence, and will think less of you if you don't show it. Which makes it more likely that people who wouldn't otherwise show overconfidence would now show it, feeding the feedback loop. And of course people will compare themselves with the level of confidence that others will show, leading to people (especially people with self-esteem issues, or who are told are less capable, or who feel like outsiders...) feeling like a failure when they aren't.
Yes, and: some people aren't allowed to perform overconfidence. At least in the culture I'm most familiar with, overconfidence only works in your favor if you're a man who's sufficiently normative about how they perform their gender, race, ability, class, religion, and various other requirements for being allowed to take power. This is why what gets called "confidence" in a white cis man might get called "abrasiveness" if a Black cis woman says / does the exact same thing.
If you're not in that small group of men, you're in a double bind: either perform overconfidence anyway and potentially get punished for that because you're supposed to know your place; or perform underconfidence, and never get what you want or need.
And then, if you're trans, non-binary, mixed-race, disabled in ways that aren't immediately apparent to everyone, and/or have experienced class mobility (for example, grew up poor but now work in tech), you have the additional complication that two different people in the same room, at the same time might be reading you completely differently, so Alice expects you to perform overconfidence but Bob expects you to defer to male power.
If I decide to act overconfident because I think people are reading me as a gender-conforming white cis man, this could end poorly for me if they're picking up on all the subtle signals that none of those labels apply to me, exactly. It doesn't matter whether I come out as trans or not, or even if anyone consciously considers that I might be trans; if I talk long enough, I'm going to give away that my thoughts and feelings have been shaped by experiences that aren't ones cis men are expected to have.
On the other hand, if I decide to act underconfident, people who are picking up on none of the signals might ask: what's wrong with this weak man? Why doesn't he assert himself more? And if I act the amount of confidence that I actually feel, that's still going to get read as "overconfidence" by people who look at me and see someone who has no right to be confident.
It's a lot of work to track others' perception of and expectations for you all the time, for those of us who don't occupy a fixed position in the overlapping status hierarchies that define what's possible for us to do and be. I don't think it's any fun to squarely inhabit a position of low power, either.
So even if you are perfectly aware that the people around you are being overconfident, and even if you're perfectly aware of your own worth, you still have a problem. I'm not sure what's worse: not being able to see the power dynamics and always feeling like you're not good enough and need to try harder (with no reward); or being able to see the power dynamics but not being able to do anything about it. The truth doesn't set you free; sometimes it just really pisses you off.
And then there’s a state you can get into where you’ve been punished for so much for showing confidence, for so long, that you forget whether you actually lack confidence, or just have been suppressing it for so long you forgot you had it.
everyone was very welcoming and understanding and it was cool. i behaved the way i had for years. suddenly i am "abrasive" and "uncooperative" according to Vague Reports So You Can't Actually Be Told What You Are Doing Wrong.
i quit without notice. those people had promised to protect me from this exact kind of thing. i have never recovered from this injury.
I had this exact same experience at work before too after coming out as trans
Through my jobs, I've tried adjusting this level of displayed confidence, and it turns out, it's never the correct amount
I remember under one boss that I got the feedback to need less guidance and have more autonomy and leadership. I took this to heart, and only couple weeks later, he gave me the feedback that I was being uncooperative and difficult.
you cant please anyone any of the time!
edit: oh yeah, I think my boss said that I'd "gone rogue" in that discussion