The default is related to the prefix option of the configure script when nginx is compiled; here's some strange sample from Debian:
% nginx -V | & tr ' ' "\n" | fgrep -e path -e prefix
--prefix=/etc/nginx
--conf-path=/etc/nginx/nginx.conf
--error-log-path=/var/log/nginx/error.log
--http-client-body-temp-path=/var/lib/nginx/body
--http-fastcgi-temp-path=/var/lib/nginx/fastcgi
--http-log-path=/var/log/nginx/access.log
--http-proxy-temp-path=/var/lib/nginx/proxy
--http-scgi-temp-path=/var/lib/nginx/scgi
--http-uwsgi-temp-path=/var/lib/nginx/uwsgi
--lock-path=/var/lock/nginx.lock
--pid-path=/var/run/nginx.pid
Subsequently, the default value of root is set to the html directory (as per the documentation of the root directive), which happens to be within prefix , as can be verified by looking at the $document_root variable from a simple configuration file:
# printf 'server{listen 4867;return 200 $document_root\\n;}\n' \
    >/etc/nginx/conf.d/so.10674867.conf
# nginx -s reload && curl localhost:4867
/etc/nginx/html
However, evil distributions like Debian seem to modify it quite a bit, to keep you extra entertained:
% fgrep -e root -e include /etc/nginx/nginx.conf
    include /etc/nginx/mime.types;
    #include /etc/nginx/naxsi_core.rules;
    #passenger_root /usr;
    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
% fgrep -e root -e include \
    /etc/nginx/conf.d/*.conf /etc/nginx/sites-enabled/*
/etc/nginx/conf.d/so.10674867.conf:server{listen 4867;return 200 $document_root\n;}
/etc/nginx/sites-enabled/default:   root /usr/share/nginx/www;
/etc/nginx/sites-enabled/default:       # include /etc/nginx/naxsi.rules
/etc/nginx/sites-enabled/default:   #   root /usr/share/nginx/www;
/etc/nginx/sites-enabled/default:   #   include fastcgi_params;
/etc/nginx/sites-enabled/default:   # deny access to .htaccess files, if Apache's document root
/etc/nginx/sites-enabled/default:#  root html;
/etc/nginx/sites-enabled/default:#  root html;
So, on this instance of Debian, you can see that the root is finally set to /usr/share/nginx/www.
But as you saw with the sample server configuration that would serve its $document_root value over http, configuring nginx is simple enough that you can write your own configuration in a matter of a single line or two, specifying the required root to meet your exact needs.