HttpUtility.HtmlDecode()对某些字符失败

查看代码:

namespace TestHtmlDecode
{
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using System.Web;
    [TestClass]
    public class TestHtmlDecode
    {
        private string Convert(string input)
        {
            return HttpUtility.HtmlDecode(input);
        }
        [TestMethod]
        public void TestLeftBrace()
        {
            Assert.AreEqual("{", Convert("{"));
        }
        [TestMethod]
        public void TestGreaterThan()
        {
            Assert.AreEqual(">", Convert(">"));
        }
    }
}

TestGreaterThan通过，但TestLeftBrace失败(Convert返回{)。为什么会这样?