Linda wrote a really good entry for this on her page.

RJ's 21 level scale was primarily used to keep track of the Aes Sedai pecking order. This wasn't actually meant to accurately depict Nynaeve's real strength compared to Lanfear. Outliers are not present in any way besides placeholding on that 21 level list - because it doesn't really mean anything concrete. It was made by RJ to primarily keep track of deference, not absolute strength.



Thus, because the 21 level list was primarily used for weaker character, it makes sense that RJ imposed this list on the overall bell curve distribution in theory, because the 21 level list was just a way of getting at a subset of the population and it was all relative, anyway, so you could take members of the population present in that list, and place them in the overall bell curve. Being at the topmost level on the 21 level list, in other words, did not mean that some random character out there was much stronger than you. It merely meant that you had not met that stronger person, thus they were not present in the list.

It's apples and oranges. The bell curve is supposed to represent all possible reflections of strength, while the 21 level list was used as a means of keeping track who was stronger than who among the introduced characters. I have a ruler. I can use it to measure things between 0.1 inch and a foot. I cannot use it to measure anything above a foot, because that was not the purpose of its' design. The purpose of the 21 level list was made to keep track of a social hierarchy, not an absolute value measurable in the real world.

So, yes, strength falls along a bell curve. No, the 21 level list doesn't represent that bell curve. You can impose the 21 level list on the curve to get an idea over where particular members stood in the grand scheme of things, but the 21 level list does not conform in a 1 to 1 relationship absolute values of strength.

And I don't think strength means much anyway. You have multiple examples of "weaker" channelers with Talents that can outstrip certain things a stronger channeler can do, and stronger channelers that can't match the skills of weaker ones. Sex itself can determine how good you are at certain feats, so there are things that a Talent might make you truly exceptional at among the population as a whole (blocking gateways seems to be one of them).