[Nerwen]

To clarify: I'm making a guess here at what Tolkien was *trying* to say- the problem is that, taken literally, that second passage doesn't really add up at all. So I'm assuming he just hadn't thought it all out in terms of normal distribution etc. Or else he was just using stock phrases, and the statement that Elf-men were "no less than six and a half feet" is to be taken in the same sense as the preceding one that "their women were seldom less than six feet in height"; that is, a rough indication of normal height rather than a lower limit. This would explain how only "some of the great kings and leaders" were taller than this (apparent) bare minimum.

Again, though the phrasing is ambiguous enough that it's hard to be sure exactly what he did mean.