Dimension of TIFF reduced exporting by AO

[NgMatthew]

I exported a pagelayout with dimension of 70cm x 60cm to tiff. When using Export Map function in ArcMap 9.3.1, the dimension of the tiff 8859 pixels x 7088 pixels.

While I exported by AO program, the dimension of the tiff file became 8508 pixels x 6804 pixels. Why?

My code is:
            IExportTIFF pExportTIFF = new ExportTIFFClass();       
            IActiveView m_ActiveView2 = m_PageLayout as IActiveView;
            pExportTIFF.CompressionType = esriTIFFCompression.esriTIFFCompressionFax4;
           
            IExport pExporter = (IExport)pExportTIFF;
            pExporter.ExportFileName = @"C:\byProgram.tif";

            int iScreenResolution = 96;
            int iOutputResolution = 300;
            pExporter.Resolution = iOutputResolution;
            tagRECT exportRECT;
            exportRECT.left = 0;
            exportRECT.top = 0;
            exportRECT.right = m_ActiveView2.ExportFrame.right * (iOutputResolution / iScreenResolution);
            exportRECT.bottom = m_ActiveView2.ExportFrame.bottom * (iOutputResolution / iScreenResolution)
           
            IEnvelope pPixelBoundsEnv = new EnvelopeClass();
            pPixelBoundsEnv.PutCoords(exportRECT.left, exportRECT.top, exportRECT.right, exportRECT.bottom);
            pExporter.PixelBounds = pPixelBoundsEnv
            int hDc = pExporter.StartExporting();
            m_ActiveView2.Output(hDc, Convert.ToInt32(pExporter.Resolution), ref exportRECT, null, null);
            pExporter.FinishExporting();                  
            pExporter.Cleanup();
            pExporter = null;
            m_ActiveView2 = null;