Analog TV didn't really have a concept of fixed horizontal resolution. The vertical resolution was a discrete number of lines, but the horizontal resolution was not discrete: it was just whatever rate the analog signal was fluctuating at as the beam scanned across the line.

So in the analog days nothing was "mastered and broadcast at 640x480," it was mastered (on tape) and broadcast at 525-lines or 576-lines and the aspect ratio was not encoded whatsoever, it was purely a function of the shape of the picture tube.

So when it came time for a digital format, the number of vertical pixels had to be chosen such that it could encode the maximum rate of fluctuation of the signal within a given line, and apparently that was determined to be 720 columns. Presumably using 640 would have lost some amount of the analog source's vertical resolution. And since the displays were still analog tubes, there didn't need to be any scaling, the aspect ratio was still dictated by the picture tube (at least when watching on TV). Scaling for the correct aspect ratio only mattered if you were using a computer or a fixed-pixel display.

I guess arguably it gave you a slightly better fit for eventual 16x9 content, too.

Nowadays it is certainly annoying to have video files with non-square pixel aspect ratio but considering the standard was designed for tube TVs it makes more sense. I mean the average home PC in 1996 i don't think could even play a 720x480p video at 30fps anyway

Intriguing. Thanks for the writeup!

No it was usually (always) 720x480, or 720x576 in PAL land. Or really videosignal, but if you had to digitize it you did it in that resolution.