Why no setq, eval in a dir-locals?

[Colin Baxter 😺]

Hello,

To backup tex files in the same working directory, I have a .dir-locals
file containing the following lines:

#+begin_src emacs-lisp
((latex-mode . (
(kept-old-versions . 2)
(kept-new-versions . 16)
(delete-old-versions . t)
(make-backup-files . t)
(version-control . t)
)))
#+end_src
It works well.

I am curious though as to why I do not seem to need "setq" and "eval"
terms. These are needed however if I wish to put the backups in a
sub-directory:

#+begin_src emacs-lisp
((latex-mode . (
(eval setq backup-directory-alist '(("." . "sub-directory/")))
(kept-old-versions . 2)
(kept-new-versions . 16)
,,, etc.
#+end_src

This too works well.

Can anyone tell me why the two cases appear to be different.

Thank you.

Best wishes,