Well as a rule they don't use CNC's to rifle barrels and this barrel is about about 60 years old, and yes it was made that way on purpose because its cut rifled. They would have had to have drilled and reamed and then gone in with either the narrow or wide rifling head and then replace that head with the other and re-timed it to engage properly to cut the two odd sized grooves. If something had slipped up then only one groove would be out to lunch and they most likely would have culled the barrel and re-drilled it or tossed it into the scrap. I thought for a second that they may have broken off a rifling head and replaced it with a wider one by accident but all of the guys I have ever watched cut have cut 12, 3, 6 and nine o'clock. Never 12, 6, 9 and 3. All in all, if it was some sort of covered up screw up, I think it would have been cheaper to toss it back for re-drilling. Damnedest thing I've ever seen.