cool to see numberphile getting some attention here. All of Brady Haran's Channels are great.
I had a physics prof that wrote tests such that the class average was always around 60%. When we asked him about it, he said that one day he thought a student might pass through that would consistently score perfectly, and he wanted to leave room for that student to shine.
We had a biotech teacher who was obsessed with the bell curve. He wrote that tests in a way that most of us would land in the average/middle section of the curve. Little did he know, my classmates were above average, so the curve was skewed with most of the people above 85%... He was VERY mad, he did not succeed, because it would mean that he would have to ask questions that were not from the books that we were given, and we knew the books by heart ^^