Not an expert but I'd suspect it's your timber choice. The specific tree is also important as you want a clean and straight trunk. I'm Australia based so usual

Not an expert but I'd suspect it's your timber choice. The specific tree is also important as you want a clean and straight trunk. I'm Australia based so usual timber for this is; iron bark, jarrah, blackbutt and red gum. We have pine but I've never seen it used as split posts. Only round posts that have been treated.