I’m guessing, but it would logically follow that this is a callback to when computers used text-based interfaces. The old 80x25 character resolution.
So the “first line” is the top 1/25th of the viewable screen (4%).
A complete guess from me. I’ve been using computers since the late 80s and this is a new term to me too.
Correlation does not indicate causation.
Though in this case, I think it does. It really does.