Well, given that tests aren't random and only given to people who... I'm not sure even what, but surely not everybody who asks and not even everybody who shows some symptoms, at least here you have to be in pretty serious trouble to only get tested at all - I am not surprised percentage is constant, it's just how well the expectations of the testers match the real illness incidence.
I don't think one can make any conclusions from testing numbers unless there's real random mass testing. Which isn't going to happen in the US, so I have no idea what these numbers mean.