terminal.app eats SHIFT key when used with UP and DOWN arrows (but not left or right)
I'm not sure what there is to &quot;explain&quot;.  If you want them to be recognized, define them in your terminal profile.
The canonical list of ANSI keycodes I usually use is here:
https://github.com/timothybasanov/terminal-app-function-keys
 ⇧↑ \033[1;2A
 ⇧↓ \033[1;2B