I was unaware that commercial rifle producers did not follow SAAMI specifications…
The vague bore size reference has more to do with historical time period and the type of bullets used.
Yes, early (read old) 38-55 rifle bores were slightly inconsistent but modern manufactures follow SAAMI specifications and production tolerances have improved over the past 140 years.
What holds true today, regardless of caliber, is that shooters need to know their rifle’s bore size and how that relates to using lead, jacketed, and hard cast bullets.
A more interesting counterpoint would have been about chamber size and the evolution from black powder and paper patched bullets to smokeless powder and jacketed bullets. Still irrelevant when discussing modern production rifles, but at least it would have been interesting.
Starline Brass has a fantastic write up on the 38-55 case which touches on this topic.
Buffalo Bore speaks a bit to bullet diameter if you look at the 38-55 ammunition.
