Minecraft wiki entry for Before_Village_%26_Pillage, associated text: Villagers need to be inside a sphere (radius = village radius) around the village center in order to breed, but the village population counts only villagers that are inside a box (width, length = 2× village radius, height: 8 [always!]) around the village center. So with a sphere that can grow to any size, and a box that's always only 8 high, it should be apparent that there are some zones only covered by the sphere (above and below the village center).