I'ts because it is still ambient occlusion. Distance of the occlusion is defined by how deep the concave area is (in inverted mode, by how deep it is on the other side). If your planks are really thin, then the corner in the inside won't be very deep, and you will not have much distance. You can boost this slightly by increasing spread and decreasing directionality but only to some extent. Using AO for weathering effect is limited by geometric situation. If you want thin planks, yet wider wearing effect, you could tesselate those planks and use CornerEdgeToVertexColor soulburn script. Then you can clamp that generated vertex color map.
