For context, I was asking GPT to rewrite some passage in the style of various authors, like Hemingway or Waugh. I didn't even ask it for an assessment of my writing; I was given that for free.
In retrospect (this was while ago), I think the passage may have been expository in character, so perhaps it is not much a mystery why it was characterized as "academic". (When I give it samples similar to mine now, I get "formal, academic, and analytical tone". Compare this to how it characterizes an article from The Register as written in an "informal and conversational tone", in part because of the "colloquial jargon" and "pop culture references"). So my RP comparison is sensible. And there's the question of social class as well. I don't exactly speak like a construction workers, as it were.