Last night I played 3D Sonic 1 and I noticed that the levels with extra background layers (and parallax scrolling) look amazing in 3D (for instance Green Hill Zone in the Japanese version). When you switch the game to the international version the background in Green Hill Zone is just one layer and does still look good in 3D just not as amazing as with extra layers and parallax scrolling. Since Sonic 2 doesn't have backgrounds with extra layers (and parallax scrolling) in every level the 3D might not look as good. When you say Emerald Hill Zone looks great in 3D I think it's because of the extra background layers. Chemical Plant Zone has just one background layer and therefore probably doesn't look as good in 3D. Just try 3D Sonic 1 and switch between the Japanese and international version and you see what I mean. I prefer the Japanese version myself and have it as default setting, but last night I switched to the international version just for fun and noticed how different it looked in 3D.