Validate JSON Web Token (JWT) With .NET JWT Library

TokenValidator.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.IdentityModel.Tokens;
using System.Linq;
using System.Net.Http;
using System.Security.Cryptography.X509Certificates;
using System.Text;

namespace kendo_board.Authentication {
    public class TokenValidator {

        static Lazy<Dictionary<string, X509Certificate2>> Certificates = new Lazy<Dictionary<string, X509Certificate2>>(FetchGoogleCertificates);

        public static Lazy<Dictionary<string, X509Certificate2>> Certificates1 {
            get {
                return Certificates;
            }

            set {
                Certificates = value;
            }
        }

        static Dictionary<string, X509Certificate2> FetchGoogleCertificates() {
            using (var http = new HttpClient()) {
                var json = http.GetStringAsync("https://www.googleapis.com/oauth2/v1/certs").Result;

                var dictionary = JsonConvert.DeserializeObject<Dictionary<string, string>>(json);
                return dictionary.ToDictionary(x => x.Key, x => new X509Certificate2(Encoding.UTF8.GetBytes(x.Value)));
            }
        }

        static public JwtSecurityToken ValidateIdentityToken(string idToken) {
            var token = new JwtSecurityToken(idToken);
            var jwtHandler = new JwtSecurityTokenHandler();

            var certificates = Certificates1.Value;

            try {
                // Set up token validation
                var tokenValidationParameters = new TokenValidationParameters();
                tokenValidationParameters.ValidAudience = ConfigurationManager.AppSettings.Get("GoogleClientID");
                tokenValidationParameters.ValidIssuer = "accounts.google.com";
                tokenValidationParameters.IssuerSigningTokens = certificates.Values.Select(x => new X509SecurityToken(x));
                tokenValidationParameters.IssuerSigningKeys = certificates.Values.Select(x => new X509SecurityKey(x));
                tokenValidationParameters.IssuerSigningKeyResolver = (s, securityToken, identifier, parameters) =>
                {
                    return identifier.Select(x =>
                    {
                        if (!certificates.ContainsKey(x.Id))
                            return null;

                        return new X509SecurityKey(certificates[x.Id]);
                    }).First(x => x != null);
                };

                SecurityToken jwt;
                var claimsPrincipal = jwtHandler.ValidateToken(idToken, tokenValidationParameters, out jwt);
                return (JwtSecurityToken)jwt;
            }
            catch {
                return null;
            }
        }

    }
}

Hi, any idea why the lazy dictionary loading brings 3 certificates out of 4 from this uri https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com

Thanks