Multithreading on multi-core processes strange results

I am trying to verify the threading of a multi-threaded program in a virtual machine with multiple cores. I wrote C # code for it:

class Program
{
    public static int fib(int n)
    {
        if (n < 2)
            return n;

        return fib(n-1)+fib(n-2);
    }

    public static void execution(object n)
    {
        int STEP = 40;
        var start = DateTime.Now;
        int value = fib(STEP);
        var end = DateTime.Now;

        Console.WriteLine(string.Format("threads: {0}, time : {1}, start: {2}, end: {3}", n, end.Subtract(start).TotalSeconds,
            start, end));
    }

    static void Main(string[] args)
    {
        int[] threads = {1, 2, 4, 8, 16};

        for(int j=0; j<5; ++j)
        {
            for (int i = 0; i < threads[j]; ++i)
            {
                var thread = new Thread(Program.execution);
                thread.Start(threads[j]);
            }
            Thread.Sleep(60000);
        }
    }

and this is the result that I got

topics: 1, time: 4.2177734, start: 02/08/2012 7:30:13, end: 2/8/2014 7:30:18 PM

topics: 2, time: 4.1015625, start: 02/08/2012 19:31:13, end: 2/8/2014 7:31:17 PM

topics: 2, time: 4.2441407, start: 02/08/2012 19:31:13, end: 2/8/2014 7:31:18 PM

topics: 4, time: 2.0351562, start: 02/08/2012 19:32:13, end: 2/8/2014 7:32:15

topics: 4, time: 2.0527343, start: 08/02/2012 19:32:13, end: 2/8/2014 7:32:15 PM

threads: 4, time: 2.0869141, start: 02/08/2012 19:32:13, end: 2/8/2014 7:32:15 PM

topics: 4, time: 2.0898437, start: 2/8/2014 7:32:13, end: 2/8/2014 7:32:15 PM

: 8, : 3.34375, : 02.08.2012 19:33:13, : 2/8/2014 7:33:17 PM

: 8, : 3.381836, : 02.08.2014 19:33:13, : 2/8/2014 7:33:17 PM

threads: 8, : 3.3066406, : 08.02.2012 19:33:14, : 2/8/2014 7:33:17 PM

threads: 8, : 3.2451172, : 08.02.2012 19:33:14, : 2/8/2014 7:33:17 PM

: 8, : 3.4560547, : 02.08.2012 19:33:13, : 2/8/2014 7:33:17 PM

: 8, : 3.5029296, : 08.02.2012 19:33:13, : 2/8/2014 7:33:17 PM

threads: 8, : 3.2841796, : 08.02.2012 19:33:14, : 02.08.2012 7:33:17 PM

: 8, : 3.4160157, : 08.02.2012 19:33:14, : 2/8/2014 7:33:17 PM

: 16, : 5.9921875, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.4404297, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 5.3896484, : 2/8/2014 7:34:15 PM, : 2/8/2014 7:34:20 PM

: 16, : 5.9658203, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 5.9873047, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.2226563, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.1552735, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.5576172, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.5273437, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.2529297, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.2958984, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 5.8544922, : 2/8/2014 7:34:15 PM, : 2/8/2014 7:34:20 PM

: 16, : 6.3886719, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 5.7089844, : 2/8/2014 7:34:15 PM, : 2/8/2014 7:34:20 PM

: 16, : 6.7207031, : 08.02.2012 19:34:14, : 2/8/2014 7:34:20 PM

: 16, : 6.0742188, : 08.02.2012 19:34:14, : 2/8/2014 7:34:21 PM

, 4- Windows 7.

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