I was always under the impression that they relied on people who came to the tower, they did very little active recruitment and relied on people coming to the Tower, very few of these will be wilders (who are supposed to be stronger?). That way most modern Aes Sedai representation is below what would be "average".

I don't agree about the circle maths though, I always got the impression being in a circle allowed you to do things you couldn't do as 3 individuals but your combined "power" wasn't that of a single channeler who had the combined strength of the 3 in the circle.

As has been pointed out though, your assertion that the highest is exactly twice as powerful as the middle is wrong though, bell curves by their nature don't allow that kind of detail. It's very possible Rand is an outlier many standard deviations of the mean away from the mean. I imagine in actuality he's rather closer than that but it's worth noting for the calculations as it's impossible for the population to actually be a bell curve.