Vertical calculation differs from google maps

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:

Start with the linked JSFiddle and compare sphericalmercator.ll results with the bounds returned by Google Maps for the shown map dimensions, zoom, and center. Trace the latitude calculation and verify the north, south, east, and west values against map.getBounds().toJSON(); done means the calculated corner markers align with the map bounds.