Why are the j and k keys associated with forward/back, respectively and why not vice versa?

This is a multiple-part question:

1. Why is it common to see the j and k keys implemented respectively as forward and backward navigation in web apps? I think it because of Bill Joy's work on vi and the choices made by the interface designers of the ADM-3A terminal.

2. More importantly, why is j associated with forward while k is associated with back? It seems counter-intuitive to me as someone who primarily reads and writes left-to-right, top-to-bottom. If my thoughts are correct about #1, I want to understand in a detailed way why the original designers chose these associations.

I am looking for authoritative information to support answers.

Here is an example: a screenshot of a portion of the keyboard shortcuts available at https://music.youtube.com: