But something thats part of the canon doesn't have to share 100% continuity with all parts of that canon.
Nerys:
There are many examples of similar words that essentially mean the same thing but can also be used to describe something more specific, too.
The point you are getting at is that the word canon can also be used to describe works within a certain field of study and not each of those pieces of fiction don't have to refer or connect with one another.
Another way of putting it is that "canon" can be a group of literary works that are generally accepted as representing a field: "the durable canon of American short fiction" (William Styron).
However, that doesn't mean that canon cannot be a collection of one big continuous piece of work or connected as a whole either (like within the world of Star Trek).
In fact, the words "canon" and "continuity" have both been interchangeably used in many cases to describe nitpicks as a whole or within a particular chapter or story line (not just within Star Trek, but within other series, as well).
Sure, the word "canon" can be used in a unique or specific way to better describe or explain a particular sentence. However, there are a ton of words out there that basically have the same meaning and have a more added particular definition attached to it.