The bell shaped curve is called the Normal Distribution or the Gaussian Distribution. It's used in all different types of measurement. When a measured quantity is affected by random error, the result is a bell shaped curve. The average is at the center of the distribution, How wide the bell is depends on the standard deviation. Take a look at this link.
http://www.regentsprep.org/Regents/math/algtrig/ATS2/NormalLesson.htm