@Zib I’m an English teacher with years of experience with IGCSE and also mark for Cambridge.
Eng Lit marking is always problematic because it’s subjective. Yes there is a mark scheme but it requires interpretation and this is where problems can arise. When you’re marking you only get about 1 in 10 of your scripts checked for tolerance (and this can be up to 3 marks either side of the agreed upon grade for the paper by the senior examiner) so there is plenty of opportunity to have poor marking go unnoticed, unfortunately.
Something that has always bothered me with Lit marking is that it is not a requirement for teachers to have read all the set texts when they mark. This is hugely problematic because it means markers can fail to credit interesting and insightful ideas if a student is going away from the suggested answers in the mark scheme. I had a case in the past where I taught my class an uncommonly chosen set text and a few marks of the more able students were lower than expected. I asked for the papers back and it was clear to me that the marker didn’t know the text - they were going off the mark scheme ‘suggested answers’ only and the students who were able to make more subtle and unusual arguments were marked down as their answers weren’t on the mark scheme. I challenged the marks and they did indeed go up. So this may have happened in your child’s case.
However, I have also had cases where I’ve had the papers back for some suspiciously low marks and I couldn’t criticise the marking. It was clear in these instances that the student had fallen apart in the exam and written a load of nonsense, or misunderstood the question/didn’t answer the question. In one a student had written an entire essay and not used any quotations, which is a total death knell! In the moment of the exam all sorts of things can happen to even the brightest students, unfortunately!
With IGCSE, the mark scheme is quite prescriptive so if a student goes off piste, no matter how good the essay, they can’t be rewarded for what they’ve written. They have to hit all of the elements of the mark scheme and marks can start falling off quite dramatically if key elements such as evaluation and analysis are not up to scratch.
In the first instance I would ask the school to request the papers back, as a 3 grade difference is substantial. His teacher and the head of English will then look at them - they will be able to see the examiner’s comments to see why the marks were given - and decide if they think they warrant a remark. I would ask ASAP as there is a deadline for remarking.