What happens when compose String with difference sequence?

I am confused by below two segments of codes, what makes the difference?

    String s1 = new String("1") + new String("2");
    s1.intern();
    String s0 = "12";
    Assert.assertTrue(s1 != s0);
    Assert.assertTrue(s1.equals(s0));

    String s20 = new String("2") + new String("1");
    s20.intern();
    String s21 = "21";
    Assert.assertTrue(s21 == s20);
    Assert.assertTrue(s21.equals(s20));

let me explain my question, the above codes pass the test without assert exception, that means "s1 != s0" is true, "s21 == s20" is true as well! "s1 != s0" should be false in my understanding!
if i am not clear, below codes may explain well than my words:

    String s20 = new String("2") + new String("1");  // "21"
    s20.intern();
    String s21 = "21";  // "21"
    Assert.assertTrue(s21 == s20);  // equal reference
    Assert.assertTrue(s21.equals(s20));

    s20 = new String("1") + new String("2");  // "12"
    s20.intern();
    s21 = "12";  // "12"
    Assert.assertTrue(s21 != s20);  // NOT equal, why??
    Assert.assertTrue(s21.equals(s20));

Assert.assertTrue(s1 != s0);
// VS.
Assert.assertTrue(s21 == s20);