Even high schools that use average weighting systems vary tremendously (and I suspect that more than anything else is why colleges use unweighted grades). My own high school, for example, used a weighting system where is was still impossible to get more than 100 (we used a 100 point scale). Instead, as long as you passed a course (higher than 65), you were given pack a percentage of lost points (40% for honors, 70% for AP), so a 70 in AP Bio became a 91, but a 70 in honors bio became an 82.