It's a combination of hard work, preparation, and clear thinking. Examiners are human too 1. Marking exam scripts is a very boring task. Judging grades is a subjective and difficult matter. Yes - some exam questions are occasionally hard to understand.
Poor writing makes scripts difficult to read.
What examiners DON'T want 1. Untidy work which is difficult to read. Longwinded answers which drift on and off the subject. Answers which have been written to 'impress', filled with lots of ill-digested jargon. Too much personal opinion, name-dropping, and generalisation.