I'm guessing that it's due to the differences in garnet size and density between individual coticules. Perhaps larger garnets fractured into smaller pieces show a more noticeable increase in cutting speed than smaller ones do. Here are photos of the surfaces of two of my coticules: a La Verte and a La Petite Blanche. The La Verte has much larger garnets and is a much slower and smoother hone than the latter. My guess is that slurry on the La Verte made with a diamond plate would in effect be more similar to slurry on the La Petite Blanche made normally. Perhaps when the garnets are smaller to begin with, the change in cutting speed when slurried with a diamond plate is less noticeable.
La Verte surface:
Attachment 224715
La Petite Blanche surface:
Attachment 224716