为什么异步代码中的异常被认为是未处理的？

有一个下载文件的代码，在较低级别有一个简单的循环，限制了多次下载尝试，它看起来像这样简化：

private static bool Failed = false;

static void Main(string[] args)
{
  TaskScheduler.UnobservedTaskException += (o, a) => { throw a.Exception; };
  ChapterDownload().Wait();
}

public static async Task ChapterDownload()
{
  try
  {
    var pages = Enumerable.Range(0, 20);
    var pTasks = pages.Select(page =>
    {
      return PageDownload()
      .ContinueWith(t => Console.WriteLine("{0}{1}", t.Status, t.IsFaulted));
    });
    await Task.WhenAll(pTasks.ToArray());
  }
  catch (Exception ex)
  {
    //
  }
}

public static async Task PageDownload()
{
  if (Failed)
    throw new Exception("boom");

  try
  {
    // var file = await new WebClient().DownloadDataTaskAsync(new Uri(@"http://example.com"));
    var file = await DownloadFile(new Uri(@"http://example.com"));
    if (file != null)
      throw new Exception("Restart download, downloaded file is corrupted");
  }
  catch (Exception ex)
  {
    Failed = true;
    await PageDownload();
  }
}

public static async Task<byte[]> DownloadFile(Uri uri)
{
  byte[] result;
  WebResponse response;
  var request = WebRequest.Create(uri);

  try
  {
    response = await request.GetResponseAsync();
    using (var memory = new MemoryStream())
    {
      await response.GetResponseStream().CopyToAsync(memory);
      result = memory.ToArray();
    }
  }
  catch (System.Exception ex)
  {
    return null;
  }
  if (response.ContentLength == result.LongLength)
    return result;
  return null;
}

问题是 - 在当前形式下，它发生UnobservedTaskException在 textboom上，即 显然是我的。同时，如果我将自制下载替换为webclient-UnobservedTaskException它不再发生。有什么区别？