The answer is that I don't really know.
However, I have spent some time looking at this in past years and I think I concluded that the more academic a school the more likely they are to standardise against the cohort taking the exam (as opposed to nationally) because that gives you a much wider list of scores. The theory is that a wider breakdown enables you to pick a point at which to draw the line that lessens the risk that you are drawing it at the wrong point.
I don't quite see it myself since I presume a bell curve is a bell curve even if one is a flatter bell curve but I am no statistician!
Try not to worry about it too much. (Easier said than done, I know.)