The Tower isn't skewed towards stronger or weaker women - it is skewed towards women who sought it out.

It makes sense to me that so many of the channelers that came after the Novice books were opened were stronger than the average Sister - the Tower's population never had the strongest channelers; it had females who shared certain ideals and personality types.

This would be like trying to figure out the average IQ of the American population by testing a representative sample of the American Bar Association; sure many of them will be very smart and intelligent in whichever measure would be reflected on the IQ test but the broader population of the country would contain many smarter people who never went into the legal field by the virtue of having more members.

In terms of channeler strength, another factor keeps coming to mind, in my opinion - too weak to test for the Shawl is a very different thing than being too weak to make a good Aes Sedai, so their standards need updating as well. Berowin, for example, has such a Talent in Shielding, despite her overall weakness, that you might as well throw out most of the usual estimations as to who can Shield who - she could hold Moghedien at the point in the story we saw her, and with her lack of struggle holding Nynaeve, she may be able to hold more powerful women, as well.

Here is my alternative answer: The women occupying the 50% mark (ie: the elusive average female channeler) is busy in the world raising children, running businesses and minding her own business most likely with a block preventing her from channeling if she had the ability inborn or no training to bring it out with absolutely no interest to become an Aes Sedai in the first place. Even with the Novice books open we are only seeing the segment of the population that wants to be affiliated with the White Tower.