I think it means (in the context of the film), how much they change each other, but neither of them really verbalize it, so everything is screwed up.
Jack, especially, changes a great deal on the inside, he's just outwardly keeping up the same lifestyle because it's what he knows. With Claire, it's kind of the opposite, she changes outwardly, but remains the same deep down and can eventually go back to her old life.
Just my humble interpretation. I've only seen it once so far, so there's probably someone here who can shed more light.
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