When Paging is enabled custom summaries are calculated for the current page.

protected object WebDataGrid1_CalculateCustomSummary(object sender, CustomSummaryEventArgs e)

    {
     //calculate the St.dev summary
     if (e.Summary.CustomSummaryName == "STDEV")
     {
         //calculate the sum of all values
         double sum = 0.0;
         int n = 0;
         foreach (GridRecord gr in this.WebDataGrid1.Rows)
         {
             sum += Convert.ToDouble(gr.Items[1].Value);
             ++n;
         }
         //calculate the sum of squared deviations
         double mean = sum / n;
         sum = 0;
         foreach (GridRecord gr in this.WebDataGrid1.Rows)
         {
             sum += Math.Pow(mean - Convert.ToDouble(gr.Items[1].Value), 2);
         }
         sum = sum / (n - 1);
         return Math.Round(Math.Sqrt(sum), 2);
     }
     return null;
    }