My experience with every person was that most of them overestimate what they know regardless of experience level. You just notice that more in new grads since its easier to tell when people are wrong about simple things than when they are wrong about more difficult things.

Software engineers tend to repeat the mantra "you cannot make accurate time estimates". That is true regardless of experience level, and everyone seems to be off by about the same amount. So there we have evidence that people overestimate their skills at every level, and its not that different.

This then boils down to people being generally bad at estimating their own level of ability.