Let's take a simple example. A pair of 6 sided dice. The probably of rolling a 2 is the probability of rolling 2 1's, which is 1 sixth * sixth = 1/36. The probably of rolling a 3 is the probably of rolling either a 1 and a 2 or a 2 and a 1, which is double that of rolling a pair of 1's. Rolling a 7 is most likely because there are 6 ways to roll it. And so he dice rolling is biased towards the center than the extremes.
