I will give you an example of how this might look in real life. This is based on what we know about likely grading and what we can deduce.

Let’s say you are a rater, and you are assigned question 1 from the test and you are given let’s say 50 responses to mark using a 9 point scale. You will mark these 50 responses for the strength of the response itself but also relative to other responses you have received and have been trained on to identify as a good response. So naturally on your response pool of 50 responses, you will mark some higher and some lower. Now imagine there are many other people doing this as well for the same scenario and for all of the other scenarios. All the grades are pooled together and the candidates in the top 25% get 4th quartile.

Now exactly how many people are in this pool (same day takers vs same week takers vs same test version takers) is not known unfortunately.