I still say the ridges are a result of the lack of ultraviolet exposure due to an Earthlike ozone layer on Romulus, possibly emerging unexpectedly in the mid-2300s due to pollution or through a non-anthropocentric (vulcanocentric, whatever) climate change favorable to ozone-producing plant life. In other words, the ridges are analogous to the human vitamin D deficiency, rickets. Vulcans no doubt are capable of absorbing large amounts of UV radiation, and probably need it for proper biological function. Extreme examples of Vulcan rickets, resulting from complete isolation from UV, can be found beneath the frozen side of Remus. They're so messed up their cartilage doesn't form correctly, overgrowing in the ears and not growing enough in the nose, and their nictitating membranes don't even work anymore.
This begs the question why the Romulans wouldn't take the obvious measures to avoid the deficiency, namely artificial UV light sources in the home. Political statement? Who knows.
At the end of the day, it's a stupid explanation for a stupid inconsistency. Hopefully, all Romulans in the future will simply be shown with smooth foreheads, like in Trek 11. It does
make some sense out of the Remans, though.