Media Width for Rowling

I’m a little confused by the media width on this theme.

The theme description page says “Featured Images are displayed as thumbnails above the post title. For optimal results, select a Featured Image at least 1200px wide.” https://wordpress.com/themes/rowling

The CSS editor in JetPack says the media width is 616 pixels “The default content width for the Rowling theme is 616 pixels.”

When I inspect the featured image element in the browser it appears to be 817 x 471 pixels.

Which one is the correct size for standard images? What is the aspect ratio?

Thanks.