Can someone explain the math here? How is he generating the histogram from just the mean and the range?

UPDATE

Found this:

"Range is always a normal distribution, with the lower number being two standard deviations below the mean, and the upper number two standard deviations above. Nothing fancier is possible, in terms of input probability distributions."

So the range is two std dev from the mean. He also mentions somewhere early that he's assuming the input range is 95% confident.

If I am understanding this correctly, I have a second question: if he has the mean, and the std dev, what is the purpose of the monte carlo simulation exactly? Can't you just subdivide your range, and compute frequency using the gaussian function?