Oh that's why. I think I remember a long time ago that LCD screens can't scale images or automatically try to fit in a 4:3 ratio in its screen. I kinda wish the programming team at GOG could at least warn us of the possible screen compatibility issues with such old titles at their information page. Do you think it's possible they could customize the code to fit all screens with this configuration?
The warning would be nice. I realized after I bought Syberia that my otherwise awesome monitor can't display 4:3 at all. Any video input, regardless of resolution, is automatically stretched across the screen.
On a positive note: This problem forces me to buy a new, bigger monitor that supports 4:3. I'm going to give the old one to my mother as a birthday gift.