I'm sure someone that knows more than me about cctv images will fill in the details but......because of the speed of the person being filmed you will get overlaying of the image, as you do with webcam. The cameras only record at a certain frames per second, probably 30, so each shot can be 'ghosting' because of the speed of camera/ speed of person. You can do the same with an ordinary camera set at low shutter speed whilst photography a fast runner.
