American chocolates (like Hershey's) tend to taste more sour because they have more sugar and less cacao. The milk also tends to have a more butyric acid taste for some reason. Perhaps the difference is how the milk is treated, but those are trade secrets.
IIRC, this "accident" is simply because chocolate producers figured out it's cheaper to make the milk slightly taste like vomit (something the Americans apparently didn't mind too much) than cooling it properly. I think nowadays that wouldn't really fly but back then cheaper chocolate was maybe so desirable that consumers didn't mind the weird taste.
Adam Ragusea did a great video about the history of Hershey's and how that flavor probably came to be so prevalent. Very worth the watch. https://youtu.be/J44svaQc5WY