Energy and nutrients are not evenly distributed in the ocean, and sea creatures have evolved to zero-in on areas of their habitat which will be most conducive for the survival, growth and reproduction. The clustering of sea life around these hot spots of suitable habitat happens from the shallowest to the deepest part of the ocean. Similar observations can be made in the home aquarium.
A population of aquarium fish will congregate where the food is most often dispensed, Sycon sponges proliferate on the undersides and dark areas of the reef scape and sump and certain animals will grow in higher density where the flow is strongest or where the light intensity is most suitable. Of course the clustering pattern is not naturally seen in aquarium corals which are artificially placed on the live rock, but perhaps this video from Monterey Bay Aquarium Research Institute can make some reefers pay more attention about the clustering of similar corals in specific areas of the aquarium.