Let's take for example the Blender SE site.
The UI makes it clear that a Meta is an accessory to the main site:
(also the word "main" makes a point on its own).
The "what is meta" page says:
Meta Blender Stack Exchange is the part of the site where users discuss the workings and policies of Blender Stack Exchange rather than discussing blender itself.
However, the URL is
blender.meta.stackexchange.com, which makes it look like it's "the Blender chapter of Meta.SE", rather than "the meta part of Blender.SE".
Was this just a random choice, a choice dictated by technical needs, or does it have a more profound meaning?