How does RocFall apply standard deviation to seeder variables?

Consider an analysis for a rock weighing 500kg, with a standard deviation of 10kg.  RocFall performs a Monte-Carlo simulation and samples from that distribution.  For every simulation, the distribution is sampled to get a value of the rock mass.  In general, 99% of all values in a normally distributed random variable lie between 3 standard deviations on either side of the mean.  67% of all values lie within one standard deviation.  So, for this example, 99% of the values will lie within 470kg and 530kg, with the majority (67%) being between 490kg and 510kg.