A test can measure how well someone recognizes the examiner’s preferred kind of evidence.

Every examiner has a preferred form of answer, and a large part of doing well is learning what it looks like. The structure, the vocabulary, the kind of evidence that counts, all conventions rather than facts about the subject, and students who arrive already knowing them have an advantage that has nothing to do with understanding.

When preparing for anything assessed, ask explicitly what a good answer looks like. If you set assessments, say it plainly rather than treating it as something able students infer. Making the convention explicit is one of the cheapest fairness measures available.