Vertical calculation differs from google maps

[jantimon]

I am trying to calculate the coords of the (north east and south west) edges of a map with a given height and width:

https://jsfiddle.net/0umus671/

<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false"></script>
<script src="https://unpkg.com/sphericalmercator@1.0.5"></script>
<div id="map_canvas"></div>
<script>
  var mapEl = document.getElementById('map_canvas');
  var map;
  var mapHeight = 200;
  var mapWidth = 500;
  var mapZoom = 10;
  var mapCenter = [47.376887, 8.541694];

  mapEl.style.width = mapWidth + 'px';
  mapEl.style.height = mapHeight + 'px';


  var sphericalmercator = new SphericalMercator();
  var ll0 = sphericalmercator.ll([0, 0], mapZoom);
  var ll1 = sphericalmercator.ll([mapWidth, mapHeight], mapZoom);
  var latDelta = Math.abs(ll0[1] - ll1[1]);
  var lngDelta = Math.abs(ll0[0] - ll1[0]);

  var south = mapCenter[0] - latDelta / 2
  var north = mapCenter[0] + latDelta / 2
  var west = mapCenter[1] - lngDelta / 2
  var east = mapCenter[1] + lngDelta / 2;
  
  var northWestMarker = [north, west];
  var southEastMarker = [south, east];

  function initialize() {
    var mapOptions = {
      zoom: mapZoom,
      center: new google.maps.LatLng(mapCenter[0], mapCenter[1]),
      mapTypeId: google.maps.MapTypeId.ROADMAP
    };
    map = new google.maps.Map(document.getElementById('map_canvas'),
      mapOptions);
    new google.maps.Marker({
      map: map,
      position: new google.maps.LatLng(northWestMarker[0], northWestMarker[1])
    });
    new google.maps.Marker({
      map: map,
      position: new google.maps.LatLng(southEastMarker[0], southEastMarker[1])
    })
    setTimeout(() => {
     
     // Should return the same result:
     console.log(map.getBounds().toJSON())
     console.log({south, west, north, east})

    }, 500)
  }

  google.maps.event.addDomListener(window, 'load', initialize);
  

</script>

Unfortunately the latitude calculation is wrong and the elements are not in the north/south corners: