奇怪的C#动态行为

在下面的代码中,有两个调用,一个调用obj1,一个调用obj2,但只有一个调用正确执行.似乎局部变量类型是原因,但也应该可以从IDynamicTarget访问“Hello”,因为它扩展了IDynamicTargetBase.

namespace DynamicTesting
{
    interface IDynamicTargetBase
    {
        string Hello(int a);
    }

    interface IDynamicTarget : IDynamicTargetBase
    {
    }

    class DynamicTarget : IDynamicTarget
    {
        public string Hello(int a)
        {
            return "Hello!";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            dynamic a = 123;

            IDynamicTargetBase obj1 = new DynamicTarget();
            obj1.Hello(a);  // This works just fine

            IDynamicTarget obj2 = new DynamicTarget();
            obj2.Hello(a); // RuntimeBinderException "No overload for method 'Hello' takes '1' arguments"
        }
    }
}