How many of those 300k and 3M are registered users?
Do you have any insight as to why? My first guess is that it's non-HNers who stumble across it because of a google result or similar.
I'm pretty sure it isn't one-time stumbling upon the site. It's that most regular HN readers don't log in.
I really, really enjoy the comments on this site.
Especially because HN shows no summary or anything of the linked article: I'm not going to click on a link if the only information about it that I have is an (often purposefully) abstruse title and a domain name. Once there are a few comments that give me an actual impression of what the article is going to be, then maybe I'll go back and actually read it.