Inconsistent behaviour drawing a route between two points in Google Maps v3

var map = null;
var infowindow = new google.maps.InfoWindow();
var bounds = new google.maps.LatLngBounds();

//The list of points to be connected
var markers = [{
  "title": 'Duero',
  "lat": '40.480243',
  "lng": '-3.866172',
  "description": '1'
}, {
  "title": 'Reyes Catolicos',
  "lat": '40.47806',
  "lng": '-3.870937',
  "description": '2'
}, {
  "title": 'Guadarrama',
  "lat": '40.478998',
  "lng": '-3.878755',
  "description": '3'
}];


//    var map;
function initialize() {
  var mapOptions = {
    center: new google.maps.LatLng(
      parseFloat(markers[0].lat),
      parseFloat(markers[0].lng)),
    zoom: 15,
    mapTypeId: google.maps.MapTypeId.ROADMAP
  };
  var service = new google.maps.DirectionsService();

  var infoWindow = new google.maps.InfoWindow();
  map = new google.maps.Map(document.getElementById("map"), mapOptions);
  var lat_lng = new Array();

  var marker = new google.maps.Marker({
    position: map.getCenter(),
    map: map,
    draggable: true
  });
  bounds.extend(marker.getPosition());
  google.maps.event.addListener(marker, "click", function(evt) {
    infowindow.setContent("coord:" + marker.getPosition().toUrlValue(6));
    infowindow.open(map, marker);
  });
  for (var i = 0; i < markers.length; i++) {
    if ((i + 1) < markers.length) {
      var src = new google.maps.LatLng(parseFloat(markers[i].lat),
        parseFloat(markers[i].lng));
      createMarker(src);

      var des = new google.maps.LatLng(parseFloat(markers[i + 1].lat),
        parseFloat(markers[i + 1].lng));
      createMarker(des);
      //  poly.setPath(path);
      service.route({
        origin: src,
        destination: des,
        travelMode: google.maps.DirectionsTravelMode.DRIVING
      }, function(result, status) {
        if (status == google.maps.DirectionsStatus.OK) {
          var path = new google.maps.MVCArray();
          var poly = new google.maps.Polyline({
            map: map,
            strokeColor: '#F3443C'
          });
          for (var i = 0, len = result.routes[0].overview_path.length; i < len; i++) {
            path.push(result.routes[0].overview_path[i]);
          }
          poly.setPath(path);
          map.fitBounds(bounds);
        }
      });
    }
  }
}

function createMarker(latLng) {
  var marker = new google.maps.Marker({
    position: latLng,
    map: map,
    draggable: true
  });
  bounds.extend(marker.getPosition());
  google.maps.event.addListener(marker, "click", function(evt) {
    infowindow.setContent("coord:" + this.getPosition().toUrlValue(6));
    infowindow.open(map, this);
  });
}
google.maps.event.addDomListener(window, 'load', initialize);

html,
body,
#map {
  width: 100%;
  height: 100%;
}

<script src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk"></script>
<div id='map'></div>