Interesting and I don't know but if I were to guess I would say that it looks at the type of messages/senders that you interact with and uses that as one of its signals. If the only thing you get sent to your gmail is spam or no-reply messages and you never reply to any of it I suppose it has nothing to go on and just assumes that it is all weighted equally.

But like I say, that's 100% guesswork, based on the 50 emails in my inbox being not-spam and 16/18 emails in my spam folder being spam (and thanks for prompting me to check, btw!)