wp-content uri not being redirected

If I take a closer look into the website’s source code there is a mix of rewritten url’s and original one’s. Revolution slider url’s are all the original subdomain url’s:

<link rel='stylesheet' id='mx_skin-css'  href='http://iban-online.de/wp-content/themes/mx/css/light.css?ver=3.7' type='text/css' media='all' />
<link rel='stylesheet' id='mx_style-css'  href='http://iban-online.de/wp-content/themes/mx-child/style.css?ver=3.7' type='text/css' media='all' />
<script type='text/javascript' src='http://iban-online.de/wp-includes/js/jquery/jquery.js?ver=1.11.1'></script>
<script type='text/javascript' src='http://iban-online.de/wp-includes/js/jquery/jquery-migrate.min.js?ver=1.2.1'></script>
<script type='text/javascript' src='http://iban.hexodus.de/wp-content/plugins/revslider/rs-plugin/js/jquery.themepunch.tools.min.js?ver=4.6.3'></script>
<script type='text/javascript' src='http://iban.hexodus.de/wp-content/plugins/revslider/rs-plugin/js/jquery.themepunch.revolution.min.js?ver=4.6.3'></script>

But also all other urls with “wp-content” directory in it are reachable under both: the subdomain and the rewritten url. It seems that “wp-content” isn’t rewritten at all or only occasionally. Is this the desired behaviour? Can someone explain me why this happens?

https://wordpress.org/plugins/wordpress-mu-domain-mapping/