Yeah, there's a couple places where they discuss this, although I am not sure which video I saw it in. It supposedly makes for dramatic improvements to airflow around the wheels.
Some of that difference has to be tread pattern, too. When you upgraded, did you upgrade to a similar style tread? That would explain the bigger difference going from the 21" low rolling resistance road tires to the AT's.
I don't have a source ready (sorry!), but I've seen here several times that Rivian had originally planned to "disallow" certain interior/exterior combos, until at some point they decided to just open it up and let people get what they want. And that CY/FE was one of the combos not allowed to...