[Masterjun]

Doesn't this miss the part where the 256x224 (8:7) output resolution gets stretched into a ~4:3 (actually 64:49) image?

The SNES has a dot rate of ~5.37 MHz which is slower than the square pixel rate defined by the ATSC standards of ~6.13 MHz. It's exactly 8/7 slower, so pixels are stretched horizontally by 8/7, causing the 8:7 resolution to be stretched to (8/7)*(8/7)=64/49, which is close to 64:48 = 4:3.

> Result in an aspect ratio close to 4:3. This would mean 224*(4/3) = 298 visible dots.

If you consider what I mentioned, the factor would be (4/3)/(8/7) = 7/6, so they would have to choose something closer to 224*(7/6) = 261.33... visible dots. Which is much closer to what they chose with 256.