Just to continue the statistics discussion, I'd say that when people say bell curve, they hardly ever mean a mathematically perfect bell curve. For example, if you plot the average income in a country, you'd get something that looks pretty much like a bell curve, but the richest people make way more than twice the average. No reason it couldn't be the same in this case.

Of course, I seem to remember Jordan saying that Lanfear was as strong as it was possible for a woman to be. So it is entirely possible that he designed his world in such a way that the average is half the strength of Lanfear, but this can't be inferred from statistics alone.