Read this: FSRS increased reviews - #10 by sorata
If you do cards at higher retention you retain more of the info. On the flip side, you get to learn less new cards. Maybe for many hard decks, learning new cards is easier than retaining all of them in the long term. Maybe that but I’m not sure.
Can you take the bullet and test it? I sometimes feel increasing DR makes my CMRR value increase too. What you need here is, preferably slice off a piece of your deck and test having a DR of .90, then you can see what happens to the value CMRR gives you.
By the way, I don’t completely trust CMRR because there are unexplained behaviours I notice in it (I reported one in GitHub).