Clustering Illusion is an optical illusion that occurs when a group of objects appear to be arranged in clusters, when in reality they are randomly distributed. It is an example of a cognitive bias, where the mind perceives patterns that are not actually present. Clustering Illusion can be seen
From Wikipedia
The clustering illusion is the tendency to erroneously consider the inevitable "streaks" or "clusters" arising in small samples from random distributions to be non-random. The illusion is caused by a human tendency to underpredict the amount of variability likely to appear in a small sample of random or pseudorandom data.