I think it's about the approach that website builders have; if it's to get a quick and easy site up, WP can be a pretty good option, rather than a static straight html website, although inevitable for some with sites that need a lot of updating. So it may reveal a bigger effort by static website owners and therefore rank better for that reason, or also for that reason. Especially of shortcuts are taken to fill the dynamic website, a straight html website could have a better density and relevancy. I personally prefer a straight HTML website anytime, unless when I have too much content to handle with too many updates necessary, and, maybe, a less important website. That's my personal experience.