I think that Tolkien in that quote is intentionally using both meanings of "consistency". The story's reality must have an internal consistency; it must agree with itself. Other answers have covered that, so I won't belabor it. But also "consistency" means "texture." Tolkien labored greatly to give his stories the "texture" of reality. They are full of the same kind of details that reality is filled with. There are different languages, there are flora and fauna, there is geography, there is history. There are seasons, and climates, and microclimates. Not only do men differ from elves, dwarves, hobbits, ents, orcs, and trolls (not to mention sentient spiders and eagles), but the men differ greatly from each other. And I don't mean only that the good men and the bad men are different. Nor do I only mean that there are distinguishable varieties of good men and bad men. No, enough details are given so that the reader can easily distinguish Beregond from Denethor. Think of the characterizations of Bilbo, Frodo, Merry, Pippin, Sam, the Gaffer, Fatty Bolger, etc. They're all male hobbits, all good-guy male hobbits, and yet are all distinguishable. They have the "consistency of reality." Even the various hobbit places are different. Hobbiton, Bywater, Bree, Buckland, the Took Smials, Farmer Maggot's farm, Michael Delving -- these are all identifiably "hobbitish" and yet all different, just like they would be in reality.