StringBuffer capacity()

public static void main(String args[]){
  StringBuilder sb=new StringBuilder();

    System.out.println(sb.capacity());
    sb.append("abcabcabcabcabcabcab");
    System.out.println(sb.length());
    System.out.println(sb.capacity());
    System.out.println("-----");
    sb.append("abcabcabcabcabcabcab");
    System.out.println(sb.length());
    System.out.println(sb.capacity());
    System.out.println("-----");
    sb.trimToSize();
    System.out.println(sb.capacity());
    System.out.println(sb.length());
}

o/p is

16
20
34
-----
40
70
-----
40
40

My question is that in second line, the capacity should be 36(20+16). why is it showing as 34.