That border is the crop area currently being shown by the Crop tool. The area inside the box is the part of the image that will be kept; the area outside it is excluded. When you switch to another tool, Capture One displays only the kept area so it appears to “fill” the viewer.

If you didn’t manually crop those images, the crop was likely applied automatically from camera/lens metadata. Some mirrorless systems use automatic lens corrections that trim the image edges to reduce distortion, vignetting, and corner aberrations. That’s why it can appear only on certain photos even from the same camera body: it depends more on the lens and shooting settings than the catalog itself.

The border size is determined by how much correction/cropping is needed for that specific image, often varying with focal length and lens behavior. So yes, it can be related to parameters used at the time of shooting, especially lens focal length or lens-specific correction data.