Differences in nested coroutines

I'm getting different results with these two methods but I don't understand why.

Method 1:

private IEnumerator NestedCoroutines()
{
    IEnumerator coroutineA = SomeCouroutine(paramaterA);
    yield return StartCoroutine(coroutineA);

    IEnumerator coroutineB = SomeCouroutine(paramaterB);
    yield return StartCoroutine(coroutineB);
}

Method 2:

private IEnumerator NestedCoroutines()
{
    IEnumerator coroutineA = SomeCouroutine(paramaterA);
    while (coroutineA.MoveNext())
    {                  
        yield return null;
    }   

    IEnumerator coroutineB = SomeCouroutine(paramaterB);
    while (coroutineB .MoveNext())
    {                  
       yield return null;
    }  
}

Method 1 will go through all of coroutineA before starting coroutineB but Method 2 seems to do a little of A then a little of B then a little of A. Any idea why?