It is more related to <fileset> Ant convention upon which Maven is built.
**/* : all files in all subdirectories: see Ant Patterns.
When
**is used as the name of a directory in the pattern, it matches zero or more directories.
For example:/test/**matches all files/directories under/test/, such as/test/x.java, or/test/foo/bar/xyz.html, but not/xyz.xml.