Comment by senkora

I did some more research and I was wrong.

My source was an AMD tech talk from years ago where they mentioned keeping instruction timings the same for backwards compatibility reasons.

I believe they were talking about this for the XBox One X: https://en.wikichip.org/wiki/microsoft/scorpio_engine#Overvi... (and a similar chip for the PS4 Pro)

So basically, they upgraded and lightly enhanced the Jaguar architecture, shrunk the process (28nm -> 16nm), but otherwise kept it the same. AMD Zen was released around this time and was far superior but they decided to stick with Jaguar in order to make sure that instruction timings were kept the same.

I guess that they didn't want two hardware revisions of the same console generation running on different micro-architectures, but they were okay switching the micro-architecture for the next console generation.