Psychology
You think your nerves are written all over your face. They almost never are.
In a 1998 study, people who had just told a lie guessed that far more observers could tell they were lying than actually could, and people forced to taste something disgusting believed their disgust was visible on video far more than it was. Thomas Gilovich, Kenneth Savitsky and Victoria Husted Medvec called this the illusion of transparency: we anchor so hard on our own churning internal state that we assume it leaks onto our face.