I am inclined to doubt theories of the flavor that they just built them better because of more moral fiber or something back in the day; but I would be curious about the impact of improved industrial process control and achievability of tighter tolerances at acceptable costs.
You can aspire to build barely to spec; but if your tolerances are all file-to-fit tier unless you pay for heroic master craftsmanship there’s likely to be an incentive to put your actual target somewhat north of the spec just to keep the amount of testing required to keep abjectly unsuitable units from reaching customers and the number that need to be scrapped down.
If your tolerances are quite tight and behavior nicely dialed in you can put your target a lot closer to the spec because you can be more confident that product will actually land where you aim it.
The latter case is substantially better overall; less testing, less scrap, fewer issues in situations where sloppy tolerances can interact badly between parts and cause mechanical wear or lightbulbs that have the wrong resistance because the filament is right but one of the support wires is wrong; but would presumably generate fewer heroic outliers that were on the “+20%” side of “+/-20” tolerances.
