I've read that it has to do with the nasty chicken houses in the US that are so filthy. The eggs are power washed to remove contaminants. This also removes a protective coating on the eggs, making them more porous, and more susceptible to bacterial contamination after the fact.
So you've got two things going on. More sick mama hens in the US because they aren't universally vaccinated. So eggs come out with some contamination. And also shells that don't protect against contamination because they have been washed.
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