Hang on, if the map r the same, does that mean that the game will make it look exactly the same? No, because of the Frontier, so it should be somewhat different, regardless of what the sugimori art of the whole of sinnoh looks like....
Actually...yes. It does. Look at this. That was the map of Hoenn released with R/S.
Now, when Emerald came out...
They re-made the map so to compensate for the changes they made (that sample is kinda small, but look. You can see the battle frontier and everything.)
Now, look, compare the New and Old Sinnoh Map:
[Old, D/P Map]
[New, Platinum Map] (It's a bit blurry, but I scaled it so it's the same size as the old one...)
-Look a bit to the top-left of the cloud over Twinleaf town. You can see snow on the trees. There's a bit of snow in Twinleaf town now, correct? So they changed the map to make it more snowy.
-Look at the Team Galactic Base in Veilstone. It looks different, right? Well the Galactic HQ is different in Platinum, so they changed the map accordingly.
-And than there's the obvious one, the giant portal above the Spear Pillar, which is the entrance to the Torn World. They changed the map to add that.
Well, as you see, they always edit the map to accommodate new changes in the 3rd game.
Although (the outside of) Stark Mountain appears completely unchanged in the New Map.
However, Stark Mountain IS a volcano. Who says it can't look a bit different inside of it? It's also possible the mini-storyline in Stark Mountain is different. However, in the looks and positioning department... It's completely the same.