Why is a Variable declared as a private class member, able to change its value without invoking its object.property ?

A question was asked about “the difference between Variable and Property”. In response to Mr. Patel’s answer. I pose a follow up question. “ Why am I allowed to make a change to the variable’s value, without invoking the property, when the Varible is still declared as Private?. The modified problem is listed below.

using System;
namespace Advanced_
{
class TestClass
{
private string x = “Test Test”;
void TestFun()
{
Console.WriteLine(x);
}

    public string ChangeName
    {
        get { return x; }
        set { x = value; }
    }
    // Application Execution begins Here! 
    static void Main(string[] args)
    {
        // Class instantiation / Objection creation begins
        TestClass obj = new TestClass();
        // First Method Call/ Invocation to Object 'TestFun' with original string value
        obj.TestFun();
        // Second Method Call/ Invocation to Object 'TestFun' with modified string value
        // Note Property is not called.
            obj.x = "Test Failure";
            obj.TestFun();
        //obj.ChangeName = "Property Testing ";
    }