I reckon a decept part of that is due to American English vs. British English.

A great example of this is the Korean War, where a British brigadier in an extremely difficult situation told an American general "Things are a bit sticky, sir" - who interpreted it as "Could be better, but we're holding the line". The misunderstanding resulted in 500 dead and captured.

FWIW I’m not quite convinced there’s that much of a dialectical divide: “Not bad,” “he’s not wrong,” etc. sound entirely natural to me in American English.

The main difference may be in the range of meanings.

In a scale of 0 to 10 where "bad" is 0, one side will take "not bad" as a 4~5 while the other side meant it as 7~8.

I'd argue there's few Americans I flat out couldn't understand, even if it sounds like they're putting their words through a blender. And I say that having lived all over the country, Northeast, Midwest, West, and deep South. Accents can be thick but they're largely intelligible. Unlike, say, the Scots.

Especially compared to a language like German. I took 5 years of German and still didn't have a damn clue what anyone was saying if they were talking in dialect.

Is that really the same thing? We aren't just talking about understatement.

ngl