Time stamp in VB.Net

Question

Hi, I have used time stamp in vb.net like this - 

btnGenerateReport.Enabled = False
            Dim sw As Stopwatch = Stopwatch.StartNew()

getreport() */ this is my function through ehich i am loading data

sw.Stop()
            Dim ts As TimeSpan = sw.Elapsed
            Label1.Text = ts.TotalMilliseconds.ToString() + " ms for summary report"

now i am getting time in milliseconds in my label

but each time milliseconds are different means first time it is "3392.2505 ms"
and same code when i run second time it is "3696.0097 ms".

it should be same....

Please tell me is there ant error in my code or why it is different each time???

Thanks