C# LastIndexの不思議な挙動

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Text;
using NUnit.Framework;

namespace TestCodes
{
	[TestFixture]
	public class LastIndexTest
	{
		[Test]
		public void LastIndex()
		{
			Assert.AreEqual(-1, "馬".LastIndexOf("("));
			Assert.AreEqual(-1, "ゝ".LastIndexOf("("));
			Assert.AreEqual(0, "(馬".LastIndexOf("("));
			Assert.AreEqual(0, "(".LastIndexOf("("));
			Assert.AreEqual(0, "(".LastIndexOf('('));

			Assert.AreEqual(1, "(ゝ馬".LastIndexOf("("));
			Assert.AreEqual(1, "(ゝ".LastIndexOf("("));

			Assert.AreEqual(0, "(ゝ".LastIndexOf('('));
		}
	}
}

普通はこうだよな。。。

Python 2.6.6 (r266:84292, Feb 10 2011, 17:57:48)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> "馬".rfind("(")
-1
>>> "ゝ".rfind("(")
-1
>>> "(馬".rfind("(")
0
>>> "(".rfind("(")
0
>>> "(".rfind('(')
0
>>>
>>> "(ゝ馬".rfind("(")
0
>>> "(ゝ".rfind("(")
0
>>>
>>> "(ゝ".rfind('(')
0