Dear Kaj,
Thank you very much for the question!
It is hard to tell from the screenshot, but I assume that above the interior walls there are zones? If that's the case, when above/below a zone, if there is no zone (in this case the interior walls take the space), EcoDesigner marks it as exterior. But this "exterior part" is negligible for the entire building.
Please let me know if it answers your question!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT