Why are grading distributions so weird? How does the bell curve work?

The average Lone doesn't tell you much without the standard deviation.

Average is simply the value of all scores added together, then divided by number of scores. But it doesn't give a reliable picture. Let's say you have a room with 4 people in it. One is 3 years old, two are 12, and one is 64. The average age in that room is about 45, which is totally unhelpful information. But the standard deviation represents (in simple terms) how far off scores are from the average. In this case the standard deviation would be really wide.

In testing and scoring on a bell curve, the standard deviation and the mathematical fact that any distribution naturally occurring tends to fall along a curve help to create a fair grading curve.

For example, if there is a really hard test, and many students get scores in the 60s or 70s, with few getting higher scores, grading on a curve lifts the lower scores and resets them to reflect how well you did *compated to others* rather than to an arbitrary scale. This is particularly helpful if a professor made up a test that was harder than anticipated.

That is the most likely explanation for the grade/score variance.