From Apple's own website: Never before has this degree of fit and finish been applied to a phone. Take the glass inlays on the back of iPhone 5, for instance. During manufacturing, each iPhone 5 aluminum housing is photographed by two high-powered 29MP cameras. A machine then examines the images and compares them against 725 unique inlays to find the most precise match for every single iPhone.
It does sound ingenious (though if they really had their tolerances down, there wouldn't be any upside to painstakingly searching out mating parts with optimally compatible dimensional variation). It also sounds like it might e marketing BS. I bet they did this just to have something to talk about. People always fall for this kind of impressive, great-cocktail-party chatter that they don't necessarily understand, or know the importance of.
It wouldn't surprise me if this has minimal benefit, and Apple quits even doing it, after a decent interval has passed.