Due to compression and stretch over time, often used sheathed ropes, like the one in your photo, tend to lose their sheen and the tressed sheath lies flatter against the core (that's a good thing for a Shibari tyer, right?). But, it could just be down to a trick of the light or focus or resolution of the image. I didn't mean anything else by it, and apologize if I came across as critical in any way.