it's connected with how urljoin mechanic works. We need this last / to mark this as a directory because in next urljoins it would be handled as a "file" and removed from final path.
e.g.
urljoin( 'http://mozilla.org/products/firefox', 'download/latest') -> 'http://mozilla.org/products/download/latest'
urljoin( 'http://mozilla.org/products/firefox/', 'download/latest') -> 'http://mozilla.org/products/firefox/download/latest'