Did the original game not use the whole frame buffer? I get the same behavior, but if I hit F2 to get to the test menu, it does fill the whole monitor. It may be that the didn't have enough memory for the frame buffer to be 640x480, or didn't have the graphics performance for it to be that high, and used a smaller frame buffer. I'd guess that the arcade game is adjusted so that the black bars are offscreen and that you are actually seeing something narrower than 640-wide on the monitor.