The point is that, on the left, the two groups look about the same to the eye at first glance.
In left examples the numbers are off by a factor of 7/6 or less vs. in right examples the numbers are off by a factor of 2 or greater.
Adjacent-numbered pages: BP315 BP316 BP317 BP318 BP319  *  BP321 BP322 BP323 BP324 BP325
nice, traditional
two_dot_clusters [smaller | same | bigger]
Aaron David Fairbanks