I guess I am lucky. I have spawned a dozen worlds so far and haven't found a bridge yet. Most of what I have seen have been overlapping circle biomes. Of course, I haven't made it past 20 days yet either. I have had a smaller grass biome spawn in the center of a larger Forest biome almost concentrically, kinda cool. And, there are always smaller (almost 90% overlapped) biomes in there too.