....
Diane Duane described the Rihannsu as developing their own culture, re-evolving their language from older roots, etc, on their long journey from Vulcan to ch'Havran and ch'Rihan. I've sometimes thought that perhaps that effort to differentiate themselves from their Vulcan cousins never entirely ended, and resulted in some genetic manipulation at some point between Star Trek VI and TNG.
I think it's very plausible given that normal evolution doesn't work that fast.