✅ Confusion over defining a default implementation on an interface

using System;

namespace Test
{
    public interface IWidget
    {
        public string GetName { get; }
        
        public int GetNumber => 1;    
    }
    
    public class Widget : IWidget
    {
        public string GetName => "Widget";
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            Widget widget = new Widget();
            IWidget iwidget = widget;

            // both work
            Console.WriteLine(widget.GetName);
            Console.WriteLine(iwidget.GetName);
            
            // widget has an error
            // iwidget works
            Console.WriteLine(widget.GetNumber);
            Console.WriteLine(iwidget.GetNumber);
        }
    }
}