Let's compare two scenarios, with the principle that the higher one's mark is on a question, the more difficult it is to increase it further (i.e. getting a 60% to a 70% is far easier than getting a 70% to an 80%) In the first scenario, someone who is keen on perfecting their answers spends all the time of the exam answering four questions perfectly out of six. That's a 67% at best, generously assuming that each perfect answer got a 100%. In the second scenario, someone else spends the time to answer all the questions, with some answers being good (80-90%) but some being terrible (40-60%) due to not knowing anything. However, as this person answered everything, their mark might be somewhere closer to 75% or even 80%, with the strong answers balancing out the weak ones.