To expand on SirP, you might sometimes see a game playing on a 16:9 TV, but that's just the same 4:3 original game (cough superhyperturbostreetfighter7 cough) stretched to 16:9. The characters are fatter than they were designed to be, etc.
It could be any game the arcade owner wants, too. Heck, games on mame on (most) 17"-19" LCDs are running "out of spec" at 5:4 (unless you correctly set the aspect ratio setting to 5:4). [shrug]