Interesting experiment and I see what you mean, it almost looks like it was created by a spiralograph (if you remember one of these

). I also tried with a large soft brush, which is the one I use the most, and didn't notice the same type pattern. With a 10-flow/100-opacity I can keep the mouse button down and go back over to build the color up. With 100-flow/10-opacity you have to release the mouse button each time to build the color up.
It also looks like it starts out a lot lighter with the lower opacity. I guess it might depend on what brush you're using and the purpose as to which you'd use. Here is a small jpg with the results, tho don't know if you can tell much.
Thanks Doug & Syd for your replies.
Dennis