You're assuming that trackers account for a changing UA. Are there trackers doing that? I suppose a deliberate or malicious attempt to identify me and isolate my machine could account for a randomly spoofed UA. But if I am trying to hide in plain sight from tracking software, isn't this method of UA spoofing enough to misplace my machine into different tracking categories or throw them off my scent entirely? When I run Panopticlick, UA is usually among the highest number of bits of identifying information, the rest of the identifying settings and preferences are more likely to be shared, which makes a particular device blend in.