Great question, jtvd.
If I had to guess, maybe all those parts were from the same stock. No reason to believe the "stock" is just big enough for one set. Heck, they might get 100 pairs out of an industrial sized bar.

The "matching" comes during assembly when they put the pieces together and file/machine the barrel and bushing so they........uh........"match". That's the only way to make them match whether they came from the same stock or not.
Well, unless you have a finger bushing like my '70, which basically fits equally poorly regardless of the barrel.