Embed friendly theme not responsive when not embedded and viewed on mobile device

I found a strange behavior when Google Webmaster Tools told me that my site was not mobile friendly:
The used standard embed friendly theme seems not to be responsive when viewed under the not embedded URL on a mobile device. If viewed embedded, responsiveness is there on desktop and mobile, even not embedded on desktop as you can see in the screenshot. It is not, however, unembedded on mobile - but this is what google indexes.
What did I do wrong? Is there an option to be set?
Any help is highly appreciated. Thank you!
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