IMAWriter wrote:John, I'm confused. Did you mean "faster (greater) rpm", not "finer grinds?"
No, although the plot suggests that may also be true.
A finer grind setting on the grinder means the burrs are closer together. Beans must break into smaller fragments to pass through the gap between the burrs and exit down the chute. Every time you break bean fragments, smaller pieces (fines) break off as well. Finer grind settings result in more breakage, a finer grind, more fines, and produce a more bimodal distribution.
Typical espresso grinds tend to have the main peak at ~400um diameter, with a fines peak at ~40um. All other things being equal, a grind setting that produces 1000um diameter particles will produce far fewer fines, and appear unimodal on a particle size distribution plot.