I own both of the cabinets mentioned above (or owned since I sold Op Wolf).
In Op Wolf, the mirror is actually reflecting the depth of the monitor which is deeper since it sits below instead of the traditional placement in front. The image is actually reversed on the monitor so that when you view it in the mirror it looks normal. The optical illusion makes it appear as if it is actually going "past" the rear of the cabinet, when in reality it's just a reflection of the monitor at the bottom. ie. if the depth of the cabinet is 2 feet deep, and the monitor is mounted below facing up at 4 ft, the image will appear to be 4 feet deep which exceeds the back of the cabinet. Like magic!

As for the X-men game (which I still have) the reason for the mirror is so the the edges of the monitor appear to be overlapping. So one monitor (on the right) is mounted normally facing you, and the other is down below with a mirror. If you didn't have a mirror (and had them side by side on top) you would see the edges of the monitors in between the images.

Mirrors are awesome and ingenious inventions for arcade machines!
~ DeLuSioNaL