Have pondered the same thing on occasion, without really coming up with an answer.
Based on no knowledge whatsoever, I wonder whether it may be more noticeable on negs with large areas of fairly even tone... in the absence of anything "substantially contrasty" maybe the scanner is focussing on the grain rather than the image?
Obviously, there's contrast between the sculpture and the wall, but maybe the scanner found focus on the even tone of the wall before it reached the detail?