I think that is because we use the creative or right side of our brain at the same time as the analytical or left side of our brain. On days where the ideas just keep flowing, we are in right-brain or creative mode. Take the time to just write and not even think about what it is until you finish. This brainstorming will result in many works that just may surprise you. Since I have come to realize that I am whole-brained, I am a much more focused writer.
Most writers probably use both sides of their brain more than others.
This, of course, doesn't speak to the inspirations and other items that add up in the equation of a good writer. But after realizing that my thought patterns go through phases, now I understand more fully why it is that I have problems making decisions. Part of them wants to do the normal and expected thing. The other part longs to just let it all hang out and do a much more unconventional and creative thing. I used to be plagued by this, for my thinking was so very split.