爲什麼等待有時創建新的線程，但有時不是？

class Program 
{ 
    static void Main(string[] args) 
    { 
     var rst = DownloadPage("http://www.baidu.com"); 
     //var rst2=GetString(); 

     Console.ReadKey(); 
    } 

    private static async Task<string> DownloadPage(string url) 
    { 
     using (var client = new HttpClient()) 
     { 
      PringMsgWithThreadId("Before await"); 
      var response = await client.GetAsync(url).ConfigureAwait(continueOnCapturedContext:false); 
      var content= await response.Content.ReadAsStringAsync(); 

      PringMsgWithThreadId(content.Substring(0, 10)); 
      PringMsgWithThreadId("After await"); 
      return content; 
     } 
    } 

    private static async Task<string> GetString() 
    { 
     PringMsgWithThreadId("Before await"); 
     var result = await GetStringAsync(); 

     PringMsgWithThreadId(result); 
     PringMsgWithThreadId("After await"); 
     return result; 
    } 

    private static Task<string> GetStringAsync() 
    { 
     var task = new Task<string>(() => 
      { 
       Thread.Sleep(1000 * 2); 
       return "string after sleep two seconds"; 
      }); 
     task.RunSynchronously(); 
     return task; 
    } 

    private static void PringMsgWithThreadId(string tag) 
    { 
     Console.WriteLine($"{tag}(ThreadId:{Thread.CurrentThread.ManagedThreadId})"); 
    } 
}

輸出時運行DownloadPage（）方法輸出：運行時爲什麼等待有時創建新的線程，但有時不是？