Ok, that makes sense.
Follow up: Why would nodes be 'Unreachable'.  Don't they need to be reachable in order to participate in validating blocks?  Or what does unreachable mean?

Hi there,
New bitcoiner here.  (Is that what we call ourselves?).  My number one question right now in trying to understand everything is: how come there aren't more nodes on this network.  Out of the 150 million or so, that own bitcoin, why wouldn't there be more nodes running it.  I would think 50,000 or so would be a reasonable number to expect given the number of computer geeks and hobbyists in the space.  Also, it's pretty easy to do so if one is interested.  Are there more nodes behind firewalls and other hidden places where they are not being counted?  If so, what is that estimate? What am I missing?