1. It reflects the ratio of horizontal active to total line length. E.g. 256 width consoles (NES/SNES/some PS1 modes) have 256 active out of 341 horizontal total = 0.75, 320 width consoles (Mega Drive, Saturn, some PS1 modes) have 320/427 = 0.75. In other words, 3/4 of the horizontal signal is actual content, the rest is blanking. OSSC preserves this: Line x5 in OSSC is 1536/2046 = 0.75.
2. The OSSC is not narrowing/removing scanlines. In Lx5 mode you will have 2 darkened lines out of 5 total. Any variation from this ratio is due to the upscaling of your screen.
3. Not sure about PS1. Afaik, most consoles have a slightly narrower aspect, if you count strictly the visible area. E.g. SNES 256/224 * 8/7 = 1.306