Scan lines are completely the result of the number of lines spread across the tube height, + the focus setting! There is a knob on all arcade monitors called Focus, which controls how sharp the line drawn by the gun is. So you can imagine if the setting is set to really sharp focus, and a low number of lines are being drawn across the screen, there will of course be room for "blank" areas between lines. Typically you want your monitor focus set to just the right balance where scan lines are minimal, but pixels not too blurry either.
So to answer efjaywel, no, its NOT generated by the game board hardware. It's purely the monitor.