My DS1 fell foul of these changes, he failed to get a C in match by 3 points and although sent for remark still missed it by 1 point. He resat and again missed by 2 points because they had changed the boundary, he sat a different exam last summer and missed it again by 2 points. Again they had changed the boundary. He took his last resat (last November) and was 1 point away and very depressed by it - thankfully that time his remark came back as a pass.
Since taking the first exam 18 months ago (foundation paper) to finally getting the pass I think the grade boundary had moved up by at least 20 marks. It just seemed so unfair that he was trying so hard and reaching the required mark each time only for them to move the boundary.