I have a CS degree from a first-tier school. My experience with exams and interviews is almost the exact opposite of yours.

In exams I don't get as nervous because I know that if I know the material then I will pass, if I don't then I won't. You can't bullshit your way past the professor/TA grading your exam.

But in interviews I know that people who are far better than me have failed[0], and that people less competent than me could get the job. So it's not my coding ability or my experience that's being measured here, it's something else. And I don't know whether I have that something else or not. Maybe I do, maybe I just don't.

[0] https://twitter.com/mxcl/status/608682016205344768?lang=en

Last but not least, after you fail an exam you can usually see it with errors underlined and often have the teacher explain them for you. If you answer interview questions in a suboptimal way, you get no feedback. You don't know what was the error you made, if any. Maybe another candidate simply read about the logic puzzle that was used.

The canned response I got was "We're sorry, but we can't currently recruit you. Please try again in half a year or so, maybe we will have some new opportunities." If you press on, you just get evasive answers.