Not to flog a dead horse, but I didn't see anyone fix this...
"Back in the day" our 15 kHz display devices expected more or less a 30 Hz interlaced NTSC-like signal. The odd frames would display 262(ish) odd numbered scan lines (1,3,5,...) The even frames would display the even numbered scan lines physically interleaved with the prior odd lines (2,4,6,...), the phosphorous in the display and the persistence of vision in our brains combined the two interlaced frames into a single 525 scan line frame at 30 Hz.
However, our 15kHz devices of the day (ST's, A8s, etc) only had 240 (or so) lines of data, so they simply output the same frame twice, and didn't include the burst telling the display to switch between odd and even frames, which caused the display device to paint the the odd and even frames on top of each other, rather than interleaving the scan lines, giving a 60 Hz frame rate with half the vertical resolution. On CRT's you get the 'blank scan line' effect, because the electron gun never physically paints the 'even' parts of the screen!
When you move to an LCD, doubler, or upscaler, each scan line gets repeated, 'filling in' the 'blank' even scan lines with a copy of the odd ones, resulting in an image where all the pixels are painted twice as high, with no vertical gaps. Whether this is an improvement is very subjective, however, it is substantially different from "no data".