Winter1979 there's a 1160 pixel wide black image on the top of your page. Placed images don't support 100% browser width. If you want this to be a banner that always spans the full browser width, you need to create a rectangle and set the image as the repeating fill for the rectangle.
Mobile browsers zoom out to show the full width of the content of the page in the browser. In your case the widest content is this 1160 pixel wide images, but that's wider than your page so your page zooms out too far and appears left aligned. If you correct the near black image at the top, the site will size as you expect in mobile browsers.