Now I know that in theory a COSC movement is supposed to be more accurate the a non-COSC version... But I can't tell you how many times down through the years I have had non-COSC watches keep near perfect time... Better in fact then many COSC watches I've owned...And yes, I had had them serviced... Example, my new Legend Diver...In three days it has gained one second...Amazing...!!! Are COSC movements over rated...???
The chronometer certification is an assurance of accuracy. That doesn't mean a non-chronometer watch cannot be accurate. A side note is that COSC is a Swiss industry certification. In fact the word chronometer has a different meaning in different countries. In the USA it is a marine clock designation. The U.S. railroad industry had a "standard watch", such as the Hamilton 992B, which were tested to a set of performance metrics, somewhat similar to the Swiss chronometer.
Any decebt Quartz movement will keep bettet time than a COSC or a METAS. My G shock keeps atomic time, is solar charged ans very durable. These certificatuons are ok, it tells you the maker cares enough to test their product and have it meet some quality standard.... ultimatelly though, its all marketing. A 4 line 5512 is justas accirate as a 5513. Really, it is.
Mfg Standards are typically higher for movements that are intended for cosc cert. Google ETA watch movements and you'll learn about the different grades made for each model. Higher grades tend to be more accurate and robust over time. This is realistically more important as real world use and maintenance affects accuracy more than anything after it leaves the factory. I have a 65yr old UG watch that is just as accurate as my brand new Breitling. Well made and well kept.