Which basically says that in a normal distribution 68% of results will be within one standard deviation of the mean, 95% will be within two standard deviations, and 99.7% will be within three. The mean is set at 50 and each 10 points would represent one standard deviation. So basically 68% of all players should be between 40 and 60 on any given tool, 95% should be between 30 and 70, and everyone will fall between 20 and 80. Theoretically, since only 99.7% would fall in that range in a normal distribution there could be a very, very small number of players who fall outside the range, but they just go ahead and throw everyone between those two numbers. Basically there should be very few 20s and very few 80s, a ton of 50s, and everyone but the select few between 30 and 70.

Hopefully that made some sense to you.