Probably all of the above, plus a big one: the output of Giga Shanghai is way higher than Fremont or Austin. Take a look at this chart from the last Tesla quarterly results.
With much higher production capacity, you can assume that even if 5-10% of cars don't pass QC, the overall output is way higher. Because they produce much more than they sell currently in Shanghai, they can have stricter QC and still meet all the delivery goals for the region. Here in the US, they're more incentivized to let QC slide in order to meet delivery goals since supply and demand is more evenly balanced.
Even in places like Texas where one can rip apart the environment to their hearts content to build a massive factory and there are tons of cheap workers available, it's still not possible to match the production output of a Chinese city like Shanghai.
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