I knew it at one point and forgot. Could it be because it's being treated as redirecting to another URL, and since the 404 page exists, it returns 200?
I'm not sure why anyone would enter anything as a 404 and want it to return 200, so it would seem like a flaw instead of a feature... to me at least.
I'd imagine there are a lot of people unknowingly doing it, while Google might think they're intentionally trying to generate a page for any request.
Maximum savings promo code: MaxSavingsAtDH