I think the nuance is most stems have a complete split seam on the back where the two pinch bolts apply clamping pressure. It's a split cylinder and, as long as the two pinch bolts are torqued to similar values, the clamping pressure should be pretty consistent on the steer tube along the length of that split cylinder.

Most stem risers are not split along the entire rear-facing edge like stems are. Most have two pinch bolts, but there's usually solid material just a short bit above the upper pinch bolt. This means the stem riser will not be clamped on the steer tube uniformly. It slides over the steer tube as a split cylinder, but the lower pinch bolt compresses that split cylinder more than the upper pinch bolt can (because the cylinder is solid, and not split, above the upper pinch bolt). So it ends up looking more like a split cone than a split cylinder. I think that's what Kontact was saying earlier -- this uneven clamping, or this "split cone" hardware if you will, will end up digging that lower edge into a soft carbon steer tube (even if it's on a small scale and not outwardly visible).