The

probability curve described by the additive effect of separate probabilities.

For example, a map of the probabilities of rolling a value 3-18 by adding 3 standard 6-sided dice together results in a bell shaped curve. Connecting the probabilities results in a shape reminicent of a bell.

There is only 1 combination possible for rolling a "3" or an "18", (1-1-1) and (6-6-6), respectively. As you calculate combinations towards the middle of the range, more combinations are possible.

This effect also likely explains why the 80/20 rule seems to apply to so many situations.