Confounding trends might exist for various reasons like: labs want people to have a day or two off, so process less on the weekends or some days of the week in spreading that weekend out.
People might interact in different risk settings over the course of a week. (E.G. For some higher risk on the job and shut in on the weekends. For others time to go out clubbing on the weekends and shut in jobs during the week).
So the 7 day running average just says: we can't disentangle to get at the true raw numbers, this is the best we can do to get useful data out of the data-feed.
PS: The only way to get truly useful data would be sufficient testing capacity to issue fully randomized tests on everyone, in a mandatory way, on a regular basis. A 'well designed survey' would probably still have biases but might approach that. To the best of my knowledge (public news media), currently most tests are probably focused on populations perceived to be at higher risk, who show visible possible symptoms, or who are obviously ill.