The 240p video mode is actually a minor corruption of 480i. Since in SD CRTs, the different components of the analog video signal essentially directly drive the electron gun, it works, and you can actually get progressive scan, 60 fps video on a standard SD CRT by sacrificing half the vertical resolution (basically, you only use the odd scanlines, but you use them twice as often).
— AVS Forum, Virtual Console and 480p
