Horological Meandering

Testing doesn't result in better accuracy for an individual watch (except to remove it from the herd being shipped out to dealers)

 

Most watches, most of the time, most companies (going out on a limb here) are constructed using a combination of mechanized and human assembly of lots of small parts. They might be partially assembled, dismantled, then reassembled, adjusted, etc. The first QC step is having GOOD PARTS!

On the other hand, JLC is testing an entire watch in its case, to ensure that magnetic and water resistance measure up, AND that the movement still runs well once cased (lots of things can go wrong in casing). I'm pretty sure they test before casing the movements ... just because it's much easier to fix the movement without having to take it apart, ruin a set of good hands, etc.

INSTALLING TEST DIALS AND HAND(S) ON BARE MOVEMENT (Grand Seiko)

MONITORING WITH VIDEOCAMERA FOR 17 DAYS

In any event and in either testing process, if a movement fails to pass its tests (and we don't know how many do), it will not be thrown out in a trashbin, but set aside or sent back so someone can rectify the problem.

If you have a watch that doesn't quite pass you can hand it to an expert who knows exactly what to do to get it to run properly. That takes time and expensive people. If you have small volumes (like many independents) you can take the time time to tweak the watch so it runs properly (if you need to). 

MANUAL CONFIRMATION OF TEST FAILURES BEING NOW CORRECTED (GS)

In a larger production setting it's perhaps more economical (and better marketing) to set up programs like the 1000 hour master control process that helps ensure problems don't reach the customer causing bad PR and expensive rework.

So to answer your question, the likelihood of a JLC Master Control reaching the customer error-free is perhaps greater (which is not to say the watch will be MORE accurate) than COSC, if only because JLC are working with the cased movement whereas COSC just gets the bare movement. Accuracy might be the same in both cases.

The more eyes look at the watch (electronic or human) and the more hands touching it, the more likely shortcomings will be noticed. But whether that exceeds the risk of damage from people handling the watch is another question.

To use an automotive analogy - people take the car to a technician and say "It's running rough and the fuel economy is bad, I think it's the fuel injection" when it might easily be ignition timing, or a potato in the tailpipe, or a half-dozen other things.

Cazalea

PRESSURE & TEMPERATURE TESTING COMPLETED WATCH (GS)

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