I guess you're European?
The Amiga PAL resolution is 320x256, while the American resolution is 320x200.
Also, AtariST and PC games were back in those days at the 320x200 resolution. It saves a hell of a lot of coding keeping a game at just one resolution when having to port it to multiple systems.
But some games, like Lionheart, I think is kept at a lower resolution to maintain speed.
