The manager of a fast-food store realizes that her staffing problems are a result of the variation in the number of customers that arrive at the store. If the same number of customers came each hour, she would know exactly how many servers to have working. It turns out that the Poisson distribution works well to describe the arrivals of customers in any given hour.

Explain why the manager has more trouble staffing the store during those hours when the average arrival rate is higher.