@Theviewer it depends on the board, hence my earlier question. I use WJEC which is marked out of 100. The marks between grades are roughly in 10s - c90% for an A star, 80% for an A and so on. If an external moderator disagrees with our marking, there would need to be a difference of 10+ marks for it to make a 2 grade difference. To differ by 10 marks is quite a big deal; in my team we often differ by a couple of marks, but that is what is deemed "within tolerance" and makes no material difference. As the lead, I tend to submit the lower mark and give that mark to the student as there is less chance of being moderated downwards.
I now AQA is marked out of 50. I haven't personally used them - my school did a few years ago and had a nightmare moderation around 10 years ago, but that's before I taught or led EPQ. However, everything is likely to be much more condensed in terms of wriggle-room. If for example the assessor marks a project as 37/50 which using WJEC's grade boundaries would be a B and a moderator thinks they have been generous by one mark in all 4 of WJEC's Assessment Objectives, the mark becomes 33 which is then a C. If it's 2 marks in each AO, you are in the D mark band.
The marking process is one which takes time to get your head around. The scheme I use has bands then within those you have descriptors to help you work out where in the band a student is. The difference between the bands is the adjectives in the mark scheme eg simple, competent, excellent.
I understand your frustration. However, as @MrsHamlet says, the board will not speak to you. The correct process is to speak to the school, specifically the EPQ lead teacher. Your student should have been given their raw score before they were submitted to the board, which it sounds like they did. The teacher should NEVER tell the students what that score translates to, for the simple reason that grade boundaries can change every year. I always tell mine that had they submitted last year, their grade would have been X but it could change this year... The mark your student received should have been internally moderated within the school. The colleague who led EPQ prior to me moderated every project but I don't have the time. I do as many as I can, but always the highest and lowest for each assessor plus any they are unsure about.
Ask whether the score your student received was internally moderated. Was the whole cohort moderated downwards? Have they read the external moderator's report? Is the school challenging it if they don't agree with it? What are they putting in place to ensure this doesn't happen again?