[ohohyodafarted]

I think the real answer to the question is, the transmitted image was designed to be in 4 by 3 format. IF you take a test card and display it with the center circle on a set that is perfectly adjusted so that the circle is perfectly round, you will see that you will be missing the corners of the image on a round tube set. If you shrink the image proportionally to see the missing corner images, you will have large blank areas on the sides and top and bottom.

The truncated top and bottom of the round crt is so as to be able to achieve the correct left to right width, and a correct top to bottom height without distortion of the picture and without a blank space above and below the image on the screen. But you will still have missing image in the 4 corners because the tube is round and not rectangular.