In a mostly arbitrary manner. Those guidelines describe a site on a broad topic where there's likely to be plenty of participation. Cooking is a lot different from particle physics in this respect, especially early on.
It's easier to explain the motivation behind each one, with the understanding that we evaluate sites mostly on a case-by-case basis:
Sites need enough new questions every day to keep people interested, there has to be something on the site to do or people get bored and go elsewhere.
Our implied 'contract' with users is that if they give us a question, they'll receive a quality, peer-reviewed answer within a reasonable amount of time.
Sites are moderated by the community, in order for that to be possible, users must be actively unlocking privileges by earning reputation. Additionally, we hold democratic moderator elections once a site graduates, the user base must be able to support this. The rep 'spread' is also a good indicator of how well a site retains new users.
Back to our implied 'contract' - most questions do warrant several peer-reviewed answers, even when there can only really be one 'correct' answer.
Sites need plenty of traffic in order to grow, most of which is going to come from search
The metrics you see there are just a general suggestion of what one of our typical healthy sites is going to look like. Some sites have a higher number of unanswered questions, others don't usually get more than 2 answers per question on average.
With really niche topics, this can vary quite a bit. The important thing is simply don't panic if those numbers look scary, we'll let you know if we see a problem.
Making that health dashboard a bit smarter is something that's on the road map for the next generation Area 51. Yes, the milestones are an artifact of our experience launching sites, but they're far from a one-size-fits-all sort of proposition.