Nosler's a member of SAAMI, so there are quite a few controls, such as consistent temperature and humidity. There's even a protocol for how the round is handled before it's inserted into the chamber of the test barrel, called "the SAAMI spin."
The shooting's done indoors, with a professional chronograph under consistent lighting, not outdoors with a $100 chronograph under varying light and temperature conditions, and a guess at how many rounds went down the barrel. There's also a record not just of the velocity of every round, but the peak pressure and the pressure curve. Oh, and how far the chronograph is in front of the muzzle.
I've visited a number of professional pressure labs, including Nosler's, and while methods vary somewhat, they're always a lot more "controlled" than what happens on shooting ranges, whether private or public.
