Thanks to both correspondents for their comments. Essentially, we agree with their points.
Bruce Arroll notices an apparent mismatch between the results for rectal bleeding combined with change in bowel habit (risk increased), and the results for rectal bleeding when combined with diarrhoea or constipation (risk unchanged — or lessened). This problem has arisen because ‘change in bowel habit’, which should mean constipation or diarrhoea, means something …