If you believe 80% of them are easy, then you're perfectly calibrated as to whether or not a question is easy, and the apparent under/overconfidence remains.
I am still confused.
You don't measure calibration by asking "Which percentage of this set of questions is easy?". You measure it by offering each question one by one and asking "Is this one easy? What about that one?".
Calibration applies to individual questions, not to aggregates. If, for some reason, you believe that 80% of the questions in the set is easy but you have no idea which ones, you are not perfectly calibrated, in fact your calibration sucks because you cannot distinguish easy and hard.
Calibration for single questions doesn't make any sense. Calibration applies to individuals, and is about how their subjective probability of being right about questions in some class relates to what proportion of the questions in that class they are right about.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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