I know what you're saying, and I agree, but I don't think that's what we're seeing. For example, my wife is currently binge-watching NCIS, and she's in season 9, which I think pre-dates 4K TV source material availability, and she noticed a substantial difference. While I'm not a fan of speculation on technical matters, if I had to guess at the reason for the surprising visual differences I would say there are strategies Netflix could be using to justify that extra $60/year. For example, they might have more 4K rendered material than we would have guessed. Or Netflix might have very effective upscaling software for HD material, and 4K subscribers are literally getting a different bit stream all of the time.
We don't have any 4K Blurays yet. Comcast is not supporting 4K for any of its own streaming content yet. Amazon Prime has some 4K content, but I haven't tested Prime yet to see if we get an effect similar to that as from Netflix. Since we have a Roku box, I'll also be checking the Roku Channel. I'll post some results soon.