Why does the String.intern() method return two different results?

I have the code like this:

String str1 = new StringBuilder("计算机").append("软件").toString();
System.out.println(str1.intern() == str1); //true

String str2 = new StringBuilder("ja").append("va").toString();
System.out.println(str2.intern() == str2); //false

String str3 = new StringBuilder("Str").append("ing").toString();
System.out.println(str3.intern() == str3); //true

I can understand why str1.intern() == str1 and str3.intern() == str3 are true, but I don't understand str2.intern() == str2. Why this is false?

My java version is: 1.8.0_73