I notice that in some arcade games on MAME like Marble Madness and Xain'd Sleena the game's attract mode and coin-up screens fill my 4:3 screen, but when you start the actual gameplay, there are vertical black bars on either side.
Thought it might just be my MAME settings, but seems to be consistent in walkthroughs on YouTube.
What was the reason for this? Could the processors not handle rendering the gameplay full-screen or was there an extra bezel/border on the monitor?