Not to put words in anyone's mouth, but i remember Wes Grant talking over at TOS about how milled uppers were "straighter" more consistently than their forged brethren. He used it just like that, putting straighter in quotations and not really delving further into exactly what it meant, but i'd take it milled uppers have more consistent tolerances upper to upper (At least, LaRue and VLTOR uppers do, which are the two he uses.).

I figure, if someone knows something about accurate ARs, it's him.