When you set camera FOV to (for example), 45 degrees, wintermute applies it to viewport verically. I mean, its view and projection transformations are set up in such manner, that vertical field of view will be 45 degrees. Since viewport is a rectangle (not a square), field of view in horizontal direction will be more than 45 degrees.
The problem is that 3DS MAX, by default, applies fov horizontally. That means, that horizontal field of view will be 45 degrees, and vertical will be less than 45. So, the image with the same FOV will be look differently in 3DS MAX and WME and prerendered backgrounds became mismathced with their collision model.
Of course, Manual FOV override solves the problem, but maybe it would be more robust to offer an option, whether to apply fov in horizontal or vertical direction?
3DS MAX has such option.