I, too, know that CBR is available for every single video on Prime, but in their backroom because that how they want it delivered (preferably at 30Mbps for 1080p), doesn't mean that Amazon will store copies of CBR on its CDN (which they don't own and have to pay for) or stream that to the browsers. If you can get CBR from elsewhere, there are two plausible answers. One, someone's using a screen recorder (maybe even downscaling from 4K); two, someone who can get their hands on submitted materials transcodes it and seeds it... I could find no way to force a web browser, Prime App, or the applet on my UHD player to pull CBR from Prime, and boy did I try!
Why do you say Amazon's VBR is rather poor, what objective metric did you use, you didn't pause and stare at every frame (that's meant to be on screen for at most 1/(24000/1001) of a second)?
Maybe I'm dumb, if so, please tell me how to get my browser to stream CBR from Prime...