As someone who can't draw a thing, there's a phenomenon I've noticed that might apply. Every time I need to draw something out, I try to hold it clearly in my head, where it looks so complete, but of course it never works. My suspicion is that instead of actually seeing the details of what I want to draw in my head, my brain has filled them in with "here be details," which makes for a nice mental image but that doesn't really play so well on paper.

I also think the same thing happens when we write software with loose mental maps. Our brains fill in the gaps with "yadda yadda yadda," to make it feel complete, but really there's nothing there at all.