The phrase I DontKnow is one of the key things I look for when giving a job interview. (In other  words, you lose points for guessing but phrasing it as if you knew). Eventually, we all become able to calibrate our coworkers (though often with much unnecessarily  wasted time). Why is it so hard for people to calibrate themselves? There is of course the tautology: you don't know what you don't know, but really some people can give you an honest assessment of what they tell you and others simply cannot. Like anything else, it is probably a developed skill, but for whatever reason (under appreciation, if not actual punishment probably) is not developed in most people. -- ChrisHyser