"this figure doesn't account for random differences in how many children people have—as well as mortality rates, sex ratios, and the probability that some adult

"this figure doesn't account for random differences in how many children people have—as well as mortality rates, sex ratios, and the probability that some adults never have children." Surely a figure that is an average of 2.1 children per woman DOES account for the probability that some adults never have children ?