Pro should be closer to XB1X, the consoles are only a year apart, no doubth RDR2 development begun on base consoles years ago but optimizing for Pro should be on par with XB1X. Something dont seem right something happned while developing on mid gen upgrades.
You really cannot look at development like that. There is so many variables that goes into a game. It could be that how RS engine works, it's just better optimized for the X1 or certain aspect of the hardware is utilize better like the CPU, GPU and memory speed. You also cannot forget about support from the OEMs, optimization of their tools, resources and things like that, how familiar the devs are with each system.