_,,,^..^,,,_ said:

Laserdisc effective chroma horizontal resolution will be (at best) around 120 lines, or about 160 pixel, when using the best SuperNTSC perfectly mastered disc, while others have less... that's at native sampling rate of 3.375MHz; when divided by four, we get 40 pixel at best... am I wrong?

You're confusing effective resolution with actual resolution.

You don't have 120 lines of chroma resolution in the file you capture. You have 360. THAT is what's getting divded by 4 here.

Saying "120 lines" in this context means that you can take that 360, resize it to 120, and then resize it back up to 360 without it looking any different (excluding noise).