Weird C# quirk

NamedParams.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        public class Base
        {
            public virtual void Foo(int a, int b) { }
        }

        public class Derived : Base
        {
            public override void Foo(int b, int a)
            {
                Console.WriteLine("a = {0}, b = {1}", a, b);
            }
        }

        static void Main(string[] args)
        {
            Derived d = new Derived();
            d.Foo(a: 1, b: 2);

            Base b = d;
            b.Foo(a: 1, b: 2);

            Console.ReadKey();
        }
    }
}