Having read the rest of the responses here, my school seems to be a bit of an anomaly, but I'll post our experience anyway in case anyone wants more points of view!
Last June we sat the AS Aspects of Narrative paper, and the exam results were completely all over the place: one person got a low C which was remarked to a very high A, someone else got a D which was remarked to a C, I got a very low B - we all got 90+% on our courseworks and were expected to all get high As. More papers were sent off for remarking but they weren't changed.
So this Jan a few of us resat it, and we all improved quite a lot - the biggest difference was someone going from an E to an A, I went from a low B to a high A (and the person who originally got a D got full marks). We definitely didn't know the texts as well as we did in June, but in the run up to the exam our teacher got us to focus on writing to tick the exam board's boxes and forget about being creative and original.
I can pretty much promise that the essays we all wrote last summer were better in terms of writing actual literature essays: however, they simply didn't fit the markscheme. This time, we all wrote quite formulaically, and although we hadn't done nearly as much work as we had for the previous one, we all did better. We're lucky in that we have a brilliant English department and our teachers were great in finding out exactly what we needed to do in the exam and showing us how. I wonder if perhaps at some other schools the actual method of answering the question wasn't taught so well, even if the content was?
Again, having said that, some people improved dramatically without doing much revision at all, after writing only a couple of practice essays. So yeah, even though we did better it does still seem pretty random (and the post above about someone getting a D without even sitting the paper seems to support this!!)