If you look under the square ones, you'll see a piece of curved metal welded to make it round on the inside. I have a 5 ft bushhog brand that I accidentally backed over a stump with and broke the curved metal piece underneath. I continued to mow, but it did not mow well at all. I had it welded it back in and was back in business mowing like normal. For whatever reason, the back really needs to be curved (one way or the other) to cut properly.
I should also add a disclaimer that this is not a heavy duty bushhog, there is no way it would cut through a 2 inch sapling.