why projectnature.text and Session["projectnature"] not changed in debug watch list but changed in UI display?

VS2008 development machine XP .net 3.5
Response.Write(SaveRequestFiles()); at the end of page_load of if(!page.ispostback)
choose detail gridview will trigger sfmProject_SelectedIndexChanged
protected void sfmProject_SelectedIndexChanged(object sender, EventArgs e)
After pressed upload button,
ProjectNature.Text = pc.ProjectNature;
in sfmProject_SelectedIndexChanged ProjectNature.Text debug can see it change from "Foundation" to "Civil"
at the end of sfmProject_SelectedIndexChanged
have
Response.Clear();
Response.Write(SaveRequestFiles()); // where SaveRequestFiles() is used by uploadify function
in debug mode, SaveRequestFiles() 's Session["ProjectNature"].ToString() ? ProjectNature.Text are "Foundation" which shown in watch list
Comment Response.Write(SaveRequestFiles()); in page_load , not break at SaveRequestFiles()
sure that all assignment of Session["ProjectNature"] assignment are do ...