The lugs are stamped with a progressive die that does 3 separate operations. The hole is punched first, then it progresses to the next station where the key is formed, then to the final hit of punching out the outside profile. On the last hit, the metal stretches around the top of the hole because it lacks the support of the material that was solid before the hole was punched.
This creates a taper between the nut and receiver face and draws the barrel slightly upward. After the barrels are head spaced to the receiver, the are checked on a vee block and indicator for run-out. To correct the run-out, they are put in an arbor press and bent into the right direction to correct the run-out to .020" or less.
Ground lugs insures that the barrel is square with the receiver,(this is if the receiver is actually square, but at least the lug is square with the nut). It's not so much about accuracy, but more about keeping things straight in line with the scope and it's adjustment range.