Optional parameter unexpected behaviour

Program.cs

using System;
using System.Linq;

namespace OptionalParameterUnexpectedBehaviour
{
    class Program
    {
        static void Main(string[] args)
        {
            var numbers = Enumerable.Range(1, 5);
            var powers = numbers.Select(Pow);
            
            Console.WriteLine($"Expected: 1,4,9,16,25");
            Console.WriteLine($"Actual: {string.Join(',', powers)}"); // 1,2,9,64,625
        }

        private static int Pow(int number, int power = 2)
        {
            return (int) Math.Pow(number, power);
        }
    }
}