AuthenticationHeaderValue与NetworkCredential的区别及两种认证方法差异原因

注：此问题并非其他类似问题的重复，我已阅读过相关问题及其答案。

方法1 #

var client = new HttpClient() { BaseAddress = new Uri("https://myurl") };
var authenticationString = "username:password"; // Obviously this will be actual username & pass
var base64EncodedAuthenticationString = Convert.ToBase64String(System.Text.ASCIIEncoding.ASCII.GetBytes(authenticationString));
client.DefaultRequestHeaders.Add("Basic", base64EncodedAuthenticationString);
HttpResponseMessage responseToken = client.GetAsync("/customer/151375").Result;
string data = responseToken.Content.ReadAsStringAsync().Result;

上述方法无法生效，经检查，response.RequestMessage.RequestUri与预期的https://myurl/customer/151375不符，返回内容为HTML。

方法2 #

HttpClientHandler httpClientHandler = new HttpClientHandler()
{
    Credentials = new NetworkCredential("username", "password")
};

var client = new HttpClient(httpClientHandler);
HttpResponseMessage response =
client.GetAsync("https://myurl/customer/151375/contacts").Result;
string data = response.Content.ReadAsStringAsync().Result;

方法2可正常生效，但方法1不行。我原本认为两种请求是一致的，但显然并非如此。请问这两种方法有何区别？为何仅方法2有效？

内容的提问来源于stack exchange，提问作者CodingYoshi