Inconsistency of -Xss and -XX:ThreadStackSize in the java launcher

gistfile1.txt

$ java -version
java version "1.6.0_25"
Java(TM) SE Runtime Environment (build 1.6.0_25-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.0-b11, mixed mode)
$ java -XX:+PrintFlagsFinal TestThreadStackSize | grep 'intx ThreadStackSize'
     intx ThreadStackSize                           = 1024            {pd product}   
$ java -Xss2m -XX:ThreadStackSize=3k -XX:+PrintFlagsFinal TestThreadStackSize | grep 'intx ThreadStackSize'
     intx ThreadStackSize                          := 3072            {pd product}        
$ java -XX:ThreadStackSize=3k -Xss2m -XX:+PrintFlagsFinal TestThreadStackSize | grep 'intx ThreadStackSize'
     intx ThreadStackSize                          := 2048            {pd product}   

=>0x00007f321801b800 JavaThread "Thread-0" [_thread_in_vm, id=26066, stack(0x00007f321d770000,0x00007f321d871000)]
// 1MB stack + 4KB guard page
  0x00000000403ba800 JavaThread "main" [_thread_blocked, id=26053, stack(0x00007f32233e9000,0x00007f32234ea000)]
// 1MB stack + 4KB guard page

=>0x00007fb14801c800 JavaThread "Thread-0" [_thread_in_vm, id=26153, stack(0x00007fb14d490000,0x00007fb14d791000)]
// 3MB stack + 4KB guard page
  0x000000004016f800 JavaThread "main" [_thread_blocked, id=26140, stack(0x00007fb153c0a000,0x00007fb153e0b000)]
// 2MB stack + 4KB guard page

=>0x00007f2ea0036800 JavaThread "Thread-0" [_thread_in_vm, id=26171, stack(0x00007f2ea649f000,0x00007f2ea66a0000)]
// 2MB stack + 4KB guard page
  0x0000000040edf800 JavaThread "main" [_thread_blocked, id=26158, stack(0x00007f2eac719000,0x00007f2eac91a000)]
// 2MB stack + 4KB guard page

TestThreadStackSize.java

import java.lang.reflect.Field;
import sun.misc.Unsafe;

public class TestThreadStackSize {
  private static void crashVM() {
    try {
      makeSegfault(getUnsafe());
    } catch (Exception e) {
      // swallow
    }
  }
  
  private static Unsafe getUnsafe() throws Exception {
    Field f = Unsafe.class.getDeclaredField("theUnsafe");
    f.setAccessible(true);
    return (Unsafe) f.get(null);
  }
  
  private static void makeSegfault(Unsafe u) throws Exception {
    u.getInt(0);
  }
  
  public static void main(String[] args) throws Exception {
    Thread t = new Thread(new Runnable() {
      public void run() {
        crashVM();
      }
    });
    t.start();
    t.join();
  }
}