The image of the robber is contained within a heptagon (7 sided shape). This is a fun play with geometry and symbology. The seven-sided-man in the land of hexagons implies some kind of transcendental property, or a movement between realms. The island of Catan is about diplomacy and trade strategy. There is something stable about this notion. This stability resonates with the symmetry of hexagons, and their ability to tessellate nicely. Notice that a heptagon could never tessellate. It can only exist alone. It is an element of chaos, a prime number, etc.