[racket] Saved continuations

I did this experiment with saved continuations in DrRacket
--------------------------------------------------
> (define saved-k #f)
(define (save-it) (call-with-composable-continuation
           (lambda (k)
           (set! saved-k k)
            0)))

> (define x 10)
(call-with-continuation-prompt
 (lambda ()
   (+ x (save-it)  ))) ; continuation is (+ x []))

(saved-k 0)
(set! x 100)
(saved-k 0)
-------------------------------------
GIVES RESULTS
10
10


-------------------------------------------


(define x 10)
(call-with-continuation-prompt
 (lambda()
   (+ (save-it) x  ))) ; continuation is (+  [] x))

(saved-k 0)
(set! x 100)
(saved-k 0)

---------------------------
GIVES RESULTS
10
100

Is this a querk of Dr. Racket or does this mean that if within a saved
continuation, a variable has not yet been reduced to a value at the
time the continuation was saved, that variable can be mutated changing
the behavior of the saved continuation?

Thanks,
Harry Spier