What? How did you get to that conclusion? That's not what the article says at all? It says Phyllis Blanchard used the (incorrect) spelling with an O (while also changing the definition of the term to something most people I think would disagree with) in a paper she wrote and nobody knows why. And it spread from there.
I think you're interpreting "Today, ExtrOvert is the most common spelling of the term in the United States." to mean it's spelled with an A elsewhere, but the author even brings up the Oxford Dictionary (UK) that says that the original spelling with an A is rare in general use. I live outside the US and I pretty much exclusively see the O-spelling.
It very clearly states that since 1918 the american spelling has been 'extrovert'. That has nothing to do with whether the A or O is correct, only that O is more common in American English.
It also says she changed the definition, that's the nature of language, it evolves. That can be through a colloquialism, a hard change (as this seems to be), or many other reasons.
I am not arguing whether it is correct or not, I am simply saying it is different.
Maybe I'm tired but this comment reads to me as if you're disagreeing with me when everything you say supports what I said? My objection/question was how you came to the conclusion it's a US/UK thing. There's no support for that in the article.
Thanks to Phyllis Blanchard ExtrOversion is the prominent spelling of the word in the United States today.
In her 1918 paper, "A Psycho-Analytic Study of August Comte" she writes:
"In order to understand the marked contract between Comte's mental attitude during his early years and that of his later life, we must keep in mind Jung's hypothesis of the two psychological types, the introvert and extrovert, -- the thinking type and the feeling type."
Not only did she change the spelling of the word, but she also changed the definition!