Maven odd behavior

[Uthpala Wettewa]

Hi,
I just started learning Maven. Few days back I executed the command to build
a project.
 mvn archetype:create -DgroupId=com.mycompany.app -DartifactId=my-app
It executed by itself created the pom.xml and the directory structure.
Recently when I executed the same command I got a menu to select an
archeype. I do not know the reason for this odd behavior of Maven. If seems
like the Users guide is not valid any more. Any suggestions.
regards,
Uthpala