First of all the BOS will look at the paper work and decide whether or not you actually deserve the illness/misadventure.
Now assuming that they do they will do two things:
1. Treat your exam normally and work out what your result would have been including your exam and moderated school assessment mark.
2. Take your exam marks out of the moderation process - moderate you school's marks in that subject and then put you into the internal marks based on your school marks and ranks.
3. Then they determine which result is best for you - one including your actual exam or one based only on your rank within your school (the BOS don't get told your trial marks - by the way there is no requirement to actually do trial exams - although I don't know any school that doesn't). All the BOS get are the final school marks as whole numbers.
The BOS will always give you what is best for you when an illness/misadventure is approved.