A good real world analogy for OP strength would be IQ. Nebulous as the very concept of IQ is, one important fact about it is that IQ also has a normal distribution. Now, if you take the IQ scores of a random population sample, it will approximate to a bell curve. But if you take a sample of "people who want to go to university", you WON'T get a bell curve.

And that's the sample we have from the Rebels. A selection of people ready to give up their former lives and attach themselves to the Tower. This is NOT a random sample. In the case of Northern Altara, it worked out to there being fewer women strong enough to attain the shawl than expected (the obvious reason is RJ wanted there to be some tension about TG. The sudden addition of 600 strong channelers to the Tower would make it unreasonably strong). Take another 1000 from there, or indeed, consider the sample we have from the Two Rivers (which is also non-random, as it was age restricted), and you get a higher than average number of women capable of being Aes Sedai (almost 100%, in fact). Take many such non-random samples from many places, and the combined numbers will likely resemble the bell curve and RJ's 62.5% more closely.