What do teachers to to guarantee that exam results express students levels?

They usually desong the exam, its questions, difficulty, challenge level within the time allowed, and within the desired results level.   The questions need to be evenly distributed across the syllabus (subject of the examination) and should vary in depth and what it takes to answer the question satisfactorily

1. So, we may have5 "classical" questions instead of10 (to observe total time required to answer these questions).
2. We may opt for20-50 (multiple choice) questions that only take a tick to pick the right answer (better coverage of the materials) ..  
3. The ease/difficult of the questions (1 or2 above) can be designed to reach the desired targets ...60% pass,10% full marks, .. etc .. etc 

All in all ... this is an art on its own, not easily done, and not done propertly in most times !