Good find, I think those would be books written in English published by German publishers. The blog post discusses how ISBNs are allocated ... specifically the ones you picture are published by Springer, which is a German company that publishes in the English language.

Considering a specific example: "Forecasting Catastrophic Events in Technology, Nature and Medicine". The website's use of "Group 978-3: German language" is a bit of a misnomer, if they had said "Group 978-3: German issued" or "German publisher" it would be clearer to users.