I believe "fast barrels" are mostly the result of fast (min. spec.) chambers. All of my custom barreled rifles have min. spec. chambers, as opposed to sloppy chambers in a lot of factory rifles. My custom barreled rifles usually reach max velocity with less powder than my factory rifles.
That's likely one reason... variations between barrels may be another reason.
David Miller, who's not exactly inept when it comes to cobbling together accurate rifles, says in the several hundred 300 WBY's he's built (all Krieger barrels, obviously with custom chambers from just a few reamers), he's seen a 200 fps variation in velocity from the "slowest" to the "fastest" of those several hundred "identical" rifles.
I think there's a lot of unknowns when it comes to this stuff and what is the obvious answer might not be the "right" answer.
