Hm. You are expecting (12 10)?

> #+begin_src scheme :var y=10 :results output
>   ((lambda (x) (display x)) '(12 y))
> #+end_src

The quote extends to the whole (parenthesized) expression, i.e.
the y is quoted too, in there, so it's the symbol y, so that
output is correct:

> #+RESULTS:
> : (12 y) 

(That's Scheme, not Babel doing it). I don't know where you
want to go to, but perhaps try:

  (list 12 y)

instead: this would make a list of whatever 12 evaluates to (this
would be 12) and y evaluates to (this would be 10).