How does exactly maven creates effective plugin configuration for the same execution ids?

I have the following plugin configuration in my project:

        <plugin>
            <groupId>net.revelc.code</groupId>
            <artifactId>impsort-maven-plugin</artifactId>
            <version>1.4.0</version>
            <configuration>
                <includes>
                    <include>AA</include>
                </includes>
            </configuration>
            <executions>
                <execution>
                    <id>the_same_id</id>
                    <phase>process-test-classes</phase>
                    <goals>
                        <goal>check</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

and the following plugin configuration in my parent pom:

        <plugin>
            <groupId>net.revelc.code</groupId>
            <artifactId>impsort-maven-plugin</artifactId>
            <version>1.5.0</version>
            <configuration>
                <lineEnding>LF</lineEnding>
                <includes>
                    <include>A</include>
                    <include>B</include>
                    <include>C</include>
                </includes>
            </configuration>
            <executions>
                <execution>
                    <id>the_same_id</id>
                    <phase>process-sources</phase>
                    <goals>
                        <goal>sort</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

When I run mvn help:effecive:pom this is the merged configuration of my plugin:

<plugins>
  <plugin>
    <groupId>net.revelc.code</groupId>
    <artifactId>impsort-maven-plugin</artifactId>
    <version>1.4.0</version>
    <executions>
      <execution>
        <id>the_same_id</id>
        <phase>process-test-classes</phase>
        <goals>
          <goal>check</goal>
          <goal>sort</goal>
        </goals>
        <configuration>
          <includes>
            <include>AA</include>
          </includes>
          <lineEnding>LF</lineEnding>
        </configuration>
      </execution>
    </executions>
    <configuration>
      <includes>
        <include>AA</include>
      </includes>
      <lineEnding>LF</lineEnding>
    </configuration>
  </plugin>

According to this the default merging behavior of the plugins configuration:

is to merge the content of the configuration element according to element name.

which is shown in the includes section as it only contains the ones from the project pom and not from the parent pom. However following that logic I don't understand why the effective-pom configuration contains two goals rather than only the one from the project configuration.