There's some kind of typographical logic to it, like alt-t is †, alt-l is ¬, alt-shift-l is |, alt-/ becomes ÷, ? and ! get upside down Spanish-style, you can probably guess where ≤≥≠± come from...

And delta is alt-J, because the band Alt-J use a delta as a logo:

https://the-little-locket-blog.tumblr.com/post/115916807689/...