중첩된 HTML 순서 목록 번호 매기기가 때때로 예기치 않은 결과를 생성하는 이유는 무엇입니까?

<ol>
    <li>one</li>
    <li>two
        <ol>
            <li>two.one</li>
            <li>two.two</li>
            <li>two.three</li>
        </ol>
    </li>
    <li>three
        <ol>
            <li>three.one</li>
            <li>three.two
                <ol>
                    <li>three.two.one</li>
                    <li>three.two.two</li>
                </ol>
            </li>
        </ol>
    </li>
    <li>four</li>
</ol>

1. one
2. two
  2.1. two.one
  2.2. two.two
  2.3. two.three
3. three
  3.1 three.one
  3.2 three.two
    3.2.1 three.two.one
    3.2.2 three.two.two
4. four

1. one
2. two
  2.1. two.one
  2.2. two.two
  2.3. two.three
2.4 three (Incorrect)
  2.1 three.one
  2.2 three.two
    2.2.1 three.two.one
    2.2.2 three.two.two
2.3 four