Assumption.

Curve used is the standard normal curve (mean = 0, standard deviation = 1).

End points used for the calculation are three standard deviations from the mean (i.e. since this the standard normal curve this means end points are -3 and +3: this interval captures 99.7% of the area under the curve.

Quantities distributed over N months where N is even will not show any one month with a peak value: months N/2 and N/2+1 will have the same value.