C Sharp :: Counting Execution Time Of A Task?

I am checking out this simple piece of code:

private void button1_Click(object sender, EventArgs e)  {
            Stopwatch sw = Stopwatch.StartNew();  
            Task Task1 = Task.Factory.StartNew(() =>  {
                    Console.WriteLine("Task1 started in : "+ " "+sw.ElapsedMilliseconds);
                    Thread.Sleep(4000);
                    Console.WriteLine("Task1 finished in : " + " " + sw.ElapsedMilliseconds);
                }
                );
        }

and the results are :

Task1 started in : 2 in milliseconds
Task1 finished in : 4015 in milliseconds.

The problem is, that if i count the time with an external device, just like the timer of my watch or my cellphone s,the total time is 4,8 or 4,9 seconds, which i wouldn t expect that to happen.

The first thing i ve thought, is that it takes the ThreadPool about 800msec or 1 sec to create the thread so as to execute this task, but even if it is so, shouldn t the stopwatch count the time precisely?So , why is there a deviation between an external timer and the stopwatch ?