When you have an existing area of cobblestone/mossy cobblestone walls (henceforth "walls") that is three (3) blocks or wider, it will be "smooth" in the middle (this is intended). If you break one of the blocks in the middle (smooth area), and then replace it with another wall, that wall and all walls below will have the "bump" (have the post in the middle), despite the ones above being "smooth". It's a bit hard to explain, but if you look at the image below it should make more sense.