As in balanced, rhythmic… think classical music, Mozart-style.
Here's a prime example of the Houstons' (you know, because two people wrote this book) Mozart-ish sentences:
Then, as if rising from the ground around us on the valley floor, I began to hear the first whispers, nearly inaudible, from all those thousands who once had lived out here, a wide, windy sound of the ghost of that life. As we began to walk, it grew to a murmur, a thin steady hum. (3.1.13)
See how steady those sentences are? They're not super long, and they're not super short—nope, they're of middling length, and if a sentence is on the long side, the Houstons makes sure to throw in a bunch of clarifying commas to break up the rush of words into small beats.
So what does this style do for the book? The risk, on one hand, is that they might bore us. But when you're writing about the internment of Japanese-Americans during World War II—in short, a super-serious topic—it's kind of a risk you have to take because the payoff is this: you sound sane, intelligent, rational, and respectable. You sound like you're on the right side of history even when history has totally maligned you.
That's key for the Houstons because that's what Jeanne most wants: to have her stories be "a way of coming to terms with the impact these years have had on [her] entire life" (Foreword, 3) and have it be a "true story" with "historical context" (Foreword, 5). She wants a balance between her emotions and objective reality—and a surefire way to achieve that is to write in a measured style.