I thought SOME ST-V games had multiple regions on the same ROM and SOME ST-V games used different ROMs per region.
Looking through the MAME source would imply they're separated out like this:
https://github.com/mamedev/mame/blob/master/src/mame/drivers/stv.cpp
Separate roms for separate regions:
Die Hard Arcade / Dynamite Deka - separate ROMs for Japan and rest of world
Puzzle & Action: Treasure Hunt / Sando-R - 3 ROMs, one is Japan only, the other two are all region
Super Major League / Final Arch - separate ROMs for USA and Japan
Sport Fishing 2 - separate ROMs for USA and Japan
Taisen Tanto-R Sashissu!! / DaeJeon! SanJeon SuJeon - separate ROMs for Japan and rest of world
Zen Nippon Pro-Wrestling Featuring Virtua - separate ROMs for Japan and Taiwan
All regions in the same ROM:
Batman Forever
Columns '97
Cotton 2
Cotton Boomerang
Critter Crusher
Tatacot
Touryuu Densetsu Elan-Doree / Elan Doree
Final Fight Revenge
Funky Head Boxers
Golden Axe - The Duel
Guardian Force
Pebble Beach - The Great Shot
Radiant Silvergun
Sea Bass Fishing
Shanghai - The Great Wall / Shanghai Triple Threat
Shienryu
Steep Slope Sliders
Soukyugurentai / Terra Diver
Suiko Enbu / Outlaws of the Lost Dynasty
Virtua Fighter Kids
Winter Heat
Virtua Fighter Remix
Decathlete
Tecmo World Cup '98 (has 2 ROMs with 2 different titles but both ROMs are all-region)
Everything else is only 1 ROM with only 1 region, mostly Japan only.
EDIT: looking into it a bit more I would suspect that Die Hard/Dynamite Deka had different ROMs because there are some significant asset differences, I guess the external building was completely changed to make it less like Nakatomi Tower and even some of the textures of the rooms within the building are changed to remove the Nakatomi logos and other references to the movie. AFAIK there aren't any gameplay differences though.
(starting at 10:44 he compares the US Saturn version to the Japanese PS2 re-release but the regional differences are the same)
As for the other games with different ROMs for different regions I'd suspect they're different because of how text-heavy those games are (the mini-games because of the instructions and the sports games because of the player information)