Processing Server XML Response

So I'm studying for an exam and I only have a quick question that has been buggin me for a while. I use AJAX to obtain a XML file to parse it and insert its values into a select element. This is the code:

      var xmlHttpObj;
                  function CreateXmlHttpRequestObject() { 
          if (window.XMLHttpRequest) {
              xmlHttpObj = new XMLHttpRequest() 
          }
          else if (window.ActiveXObject) {
              xmlHttpObj = new ActiveXObject("Microsoft.XMLHTTP")
          }
          return xmlHttpObj;          }
                  function EnviaPedido() {
          xmlHttpObj = CreateXmlHttpRequestObject();
          xmlHttpObj.open("POST", "agenda.xml", true);
          xmlHttpObj.onreadystatechange = ProcessaReposta;
          xmlHttpObj.send();          }
                  function ProcessaReposta() {
          if (xmlHttpObj.readyState == 4 && xmlHttpObj.status == 200) {
              var response = xmlHttpObj.responseText;

              var xmlDoc;

              if (window.DOMParser) {
                  parser = new DOMParser();
                  xmlDoc = parser.parseFromString(response, "text/xml");
              } else {
                  xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
                  xmlDoc.async = false;
                  xmlDoc.loadXML(response);
              }

              //Processamento DOM
              var select = document.getElementById("select");

              var centrosInvestigacao = xmlDoc.getElementsByTagName("centro_de_investigacao");

              for(i = 0; i < centrosInvestigacao.length; i++) {
                  var option = document.createElement("option");
                  option.innerHTML = centrosInvestigacao[i].childNodes[1].textContent;
                  select.appendChild(option);
              }
          }           
      }

And this is the XML that is returned:

<FCT>
  <centro_de_investigacao id="1">
    <nome>GECAD</nome>
    <local>ISEP</local>
    <classificao>Muito bom</classificao>
  </centro_de_investigacao>
  <centro_de_investigacao id="2">
    <nome>DEF</nome>
    <local>ISEP</local>
    <classificao>Bom</classificao>
  </centro_de_investigacao>
  <centro_de_investigacao id="3">
    <nome>ABC</nome>
    <local>FEUP</local>
    <classificao>Muito mau</classificao>
  </centro_de_investigacao>
</FCT>

So when I want to obtain the 'nome' field why do I have to use

option.innerHTML = centrosInvestigacao[i].childNodes[1].textContent; instead of option.innerHTML = centrosInvestigacao[i].childNodes[0].textContent;

I know it's probably a stupid question but it's starting to piss me off not knowing if this is the expected behaviour or if I'm somehow screwing this up.

Thanks.