"JDK 5 not supported with Maven" error with JDK 6

When building a project that uses Maven, an error is encountered that states that Maven and JDK 5 are not compatible. However, the JDK version that was invoked is very obviously Java 6. Worse, the Maven plugin then increments the JDK version to JDK 7, which is also available on the system.

[main] $ /usr/lib/jvm/java-6-openjdk-amd64//bin/java -cp /var/lib/jenkins/plugins/maven-plugin/WEB-INF/lib/maven-agent-1.6.jar:/usr/share/maven2/boot/classworlds.jar hudson.maven.agent.Main /usr/share/maven2/ /var/cache/jenkins/war/WEB-INF/lib/remoting-2.51.jar /var/lib/jenkins/plugins/maven-plugin/WEB-INF/lib/maven-interceptor-1.6.jar 56940
<===[JENKINS REMOTING CAPACITY]===>channel started
ERROR: JENKINS-18403 JDK 5 not supported to run Maven; retrying with slave Java and setting compile/test properties to point to /usr/lib/jvm/java-6-openjdk-amd64/
[main] $ /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java -cp /var/lib/jenkins/plugins/maven-plugin/WEB-INF/lib/maven-agent-1.6.jar:/usr/share/maven2/boot/classworlds.jar hudson.maven.agent.Main /usr/share/maven2/ /var/cache/jenkins/war/WEB-INF/lib/remoting-2.51.jar /var/lib/jenkins/plugins/maven-plugin/WEB-INF/lib/maven-interceptor-1.6.jar 34098

This is not reproducible from the command line; building a project from the same pom.xml and same maven2 build and same JDK will result in a proper build.