Probably because of the way things are scaled - the ball coordinates are all based on a 640x480 image, but the Park Generator is based on a 800x600 image (well, technically it's 600x600, I just add a border). I have to do some math to translate the coordinates, so it may not be perfect.
To be perfectly honest, I never use ball flight in-game, so I've never actually tried plugging the coordinates in to see how well they work.
