JAVA_HOME gets mangled by Maven

I'm retrofitting bunch of existing Java projects with unified Maven build. Since each project is mature and has established Ant based build all I'm using maven-antrun-plugin to execute existing build.xml as follows:

        <plugin>
            <artifactId>maven-antrun-plugin</artifactId>
            <executions>
                <execution>
                    <phase>compile</phase>
                    <configuration>
                        <tasks>
                            <ant antfile="build.xml" target="compile" />
                        </tasks>
                    </configuration>
                    <goals>
                        <goal>run</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>

When I run mvn compile build fails with this message:

[INFO] An Ant BuildException has occured: The following error occurred 
       while executing  this line:
build.xml:175: Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "C:\Java\jdk1.6.0_13\jre"

What puzzles me is

1. I have JAVA_HOME=C:\Java\jdk1.6.0_13 as part of my environment setup and when mvn.bat is executed that is exactly value I'm getting, however as you see in the error message it comes up as C:\Java\jdk1.6.0_13\jre
2. If I run ant compile everything compiles just fine

Does it mean that perhaps maven-antrun-plugin does something like set JAVA_HOME=%JAVA_HOME%\jre? I searched my batch/build files I can't find where that change occurs