English Wikipedia uses two templates on nonfree images: a copyright tag and a nonfree rationale. The copyright tag contains correct license metadata including the nonfree flag; the nonfree rationale contains partial metadata (e.g. "Fair use" as license name but no nonfree tag). As a result, sometimes (depending on which template comes first) the metadata block for the rationale template is preferred, and the image is considered free. (Example: Blade_Runner_poster.jpg)
One way to fix it would be to remove copyright metadata from rationale templates, or make sure they include the nonfree flag; according to this discussion, that's problematic. The other solution is for CommonsMetadata to consider an image nonfree if any one of its license metadata blocks has the nonfree flag.