Java thread dump prio value doesn't correspond with real thread priority on linux?

I intend to use thread priorities within my Java code. The application shall run on my Linux system:

>uname -a
Linux <host> 3.0.0-15-generic #26-Ubuntu SMP <date> x86_64 x86_64 x86_64 GNU/Linux

>java -version
java version "1.6.0_23"
OpenJDK Runtime Environment (IcedTea6 1.11pre) (6b23~pre11-0ubuntu1.11.10.1)
OpenJDK 64-Bit Server VM (build 20.0-b11, mixed mode)

After some reading in the Web I start my test-application with the following command now:

sudo java -XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=1 -jar ThreadPriorityTest.jar

The test-application consists of the following two classes:

package ch.mypackage;

public class CountingRunnable implements Runnable {

    private long count = 0;
    private boolean goOn = true;

    public long getCount() {
        return count;
    }

    public void stop() {
        goOn=false;
    }

    public void run() {
        for(long iteration=0;goOn&&iteration<Long.MAX_VALUE;++iteration) {
            ++count;
        }
    }
}


package ch.mypackage;

public class PriorizedCountingThreads {

    private static final int NUM_MILLIS_TO_COUNT_FOR = 1*60*1000;
    private static CountingRunnable[] runnables;
    private static Thread[] threads;

    public static void main(String[] args) { 
        Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
        System.out.println("MIN_PRIORITY: "+Thread.MIN_PRIORITY);
        System.out.println("MAX_PRIORITY: "+Thread.MAX_PRIORITY);
        int numPriorityLevels = (Thread.MAX_PRIORITY-Thread.MIN_PRIORITY) + 1;
        init(numPriorityLevels);
        startThreads();
        try {
            Thread.sleep(NUM_MILLIS_TO_COUNT_FOR);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        stopRunnables();
        printCounts();
    }

    private static void p