XYZ map doesn't calculate the correct tile URL

Question

I have the following code:

Proj4js.defs["EPSG:31258"] = "+proj=tmerc +lat_0=0 +lon_0=13.33333333333333 +k=1 +x_0=450000 +y_0=-5000000 +ellps=bessel +towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232 +units=m +no_defs"; +lon_0=0.0 +x_0=0.0 +y_0=0 +units=m +k=1.0 +nadgrids=@null +no_defs"
var googleProj = new OpenLayers.Projection("EPSG:900913");
var salzburgProj = new OpenLayers.Projection("EPSG:31258");
var bounds = new OpenLayers.Bounds(426249,294999,426251,295001);
var maxZoom = 19;
var minZoom = 11;
var res = [76.4370282714844,38.2185141357422,19.1092570678711,9.55462853393555,4.77731426696777,2.38865713348389,1.194328566741945,0.5971642833709725];

mapCOD = new OpenLayers.Map({
    div: "map1",
    allOverlays: true,
    maxExtent: bounds,
    projection: salzburgProj,
    displayProjection: salzburgProj,
    allowSelection: true,
    units: 'm',
    controls: [new OpenLayers.Control.PanZoomBar(), new OpenLayers.Control.Navigation()]
});

var layer = new OpenLayers.Layer.XYZ(
    layer_dir,
    "tiles/${z}/${x}/${y}.png",
    {
        zoomLevels: maxZoom,
        zoomOffset: minZoom,
        resolutions: res,
        sphericalMercator: true,
        crossOriginKeyword: null,
        buffer: 1
    });

mapCOD.addLayer(layer);

The problem is that for example, when I load the map at zoom 11, I expect to get tiles

• 11/1097/713.png
• 11/1098/713.png

Instead I get:

• 11/1044/1008.png
• 11/1044/1010.png
• 11/1043/1008.png
• 11/1043/1009.png
• ...and so on

So the way OpenLayers calculates tiles URLs seems wrong or not properly configured.

I have produced tiles with mapnik using "EPSG:31258": so my tiles have the same directory/name structure of Open Street Map, but with a different projection than the default 900913.

Where is the problem and how can I solve this?