为什么C#编译器不会对具有默认参数的方法感到困惑？

在下面的代码中SayHello()可以参考：

> SayHello()
> SayHello(字符串arg1 = null)
> SayHello(字符串arg1 = null,字符串arg2 = null)
> SayHello(string arg1 = null,string arg2 = null,string arg3 = null)

但是这段代码编译成功没有任何模糊错误.

class Program
{
    private static void SayHello()
    {
        Console.WriteLine("Hello 1");
        return;
    }

    private static void SayHello(string arg1 = null)
    {
        Console.WriteLine("Hello 2");
        return;
    }

    private static void SayHello(string arg1 = null,string arg2 = null)
    {
        Console.WriteLine("Hello 3");
        return;
    }

    private static void SayHello(string arg1 = null,string arg3 = null)
    {
        Console.WriteLine("Hello 3");
        return;
    }

    private static void Main(string[] args)
    {
        SayHello(); // SayHello() invoked,but SayHello(string arg1 = null) not invoked.
        SayHello("arg1");
        SayHello("arg1","arg2","arg3");

        // Output is:
        // Hello 1
        // Hello 2
        // Hello 3

        return;
    }
}