I omitted the version part for simplicity.
What confuse me is
1) what is jstl-api.jar for ?
2) why in the first project, it works fine without jstl-api.jar, but in the second project it uses a combination of them ?
3) For jstl.jar, the second one is clear it is from "javax.servlet.jsp", but the first one is just "jstl", sounds strange.
JSTL, like certain other JEE components, is comprised of two separate libraries: an API jar and an implementation jar.
The API jar defines the Application Programming Interface - the classes that your webapp uses to talk to the component in question. Most of these classes are Interfaces and specifically Interfaces defined by the JEE standard.
The Implementation jar does the actual work. It contains the classes which implement the API.
For JSTL, as for JSP, the API and Implementation jars are supplied as part of the web application server. The Implementation jars have no business whatever being invoked in a webapp POM, as they are appserver-specific. The API jars are not appserver-specific, and in fact, that's the very reason for having a separate API and implementation - to make webapps more "write-once/run-anywhere". In a Maven POM, the scope of the API jar should be "provided", since at run-time, the copy of the API jar that's internal to the webapp server should be used. Note that if you don't define a scope in Maven, it will default to "compile" and that can lead to trouble here.
Not everything you read on the Internet is correct, so the example that you cited is not a good example.
Loudly announcing something is true and finding out you're wrong makes you feel foolish.
Finding out you're wrong and refusing to admit it makes you LOOK foolish.