Telerik: this behavior is expected and specific to how excel works - it updates its formula on workbook open. The solution is to open and trigger update of the data file before the test runs. 

We have an option to use our Execution Extension (http://docs.telerik.com/teststudio/advanced-topics/coded-samples/general/execution-extensions) and in it to reload the Excel file before the execution of the test starts, which will solve your problem. 

Attached is the updated project (TestPRoject24Updated.zip) and built execution extension (ExecutionExtensionTAS.dll) to be placed in [Test Studio install directory]\Bin\Plugins\ folder to trigger the refresh. 

Here is the code implemented in OnInitializeDataSource(ExecutionContext executionContext) to trigger the update of the data file using Test Studio execution extension:
             public System.Data.DataTable OnInitializeDataSource(ExecutionContext executionContext)
            {
            Microsoft.Office.Interop.Excel.Application objexcel;
            Microsoft.Office.Interop.Excel.Workbook wbexcel;
            objexcel = new Microsoft.Office.Interop.Excel.Application();
            objexcel.Visible = false;
            wbexcel = objexcel.Workbooks.Open(executionContext.DeploymentDirectory.ToString()+"\\Data\\"+executionContext.Test.Source.ToString());
            wbexcel.RefreshAll();
            wbexcel.Save();
            objexcel.Workbooks.Close();
            objexcel.Quit();
            return null;
            }