[plt-scheme] Moving-ball question - My question is when ball disappeared, instantly it comes to center of window

My question is when ball disappeared, instantly it comes to center of window

;; Teachpack: draw.ss

(define-struct ball (radius x y x-dot y-dot color))
;; A Ball is (make-ball Number Number Number Number Number Symbol)

#| Template
1. Trivial case:
Ball is entirely out of bounds.  In that case, simply return true.
2. Divide:
Draw the ball, erase it, then replace it by a moved copy.
3 Conquer:
Nothing to do here.
|#

;; move-ball-until-gone : Ball -> true
;; display a ball moving until it disappears from the window
;; uses generative recursion
(define (move-ball-until-gone b)

  (cond
    ((out-of-bounds? b) true )
        (else (and
           (draw-and-clear-ball b)
           (move-ball-until-gone (move-ball b))))))

(define DELTA-T 1)      ; controls speed of simulation
(define DELAY 0.1)      ; seconds between updates

;; move-ball : Ball -> Ball
;; models the movement of a ball
(define (move-ball b)
  (make-ball (ball-radius b)
             (+ (ball-x b) (* DELTA-T (ball-x-dot b)))
             (+ (ball-y b) (* DELTA-T (ball-y-dot b)))
             (ball-x-dot b)
             (ball-y-dot b)
             (ball-color b)))

;; drawing window dimensions
(define WIDTH 300)
(define HEIGHT 300)

;; draw-and-clear-ball : Ball -> true
;; draws ball, waits a short time, then clears it
(define (draw-and-clear-ball b)
  (and (draw-solid-disk (make-posn (ball-x b) (ball-y b))
                        (ball-radius b)
                        (ball-color b))
       (sleep-for-a-while DELAY)
       (clear-solid-disk (make-posn (ball-x b) (ball-y b))
                         (ball-radius b)
                         (ball-color b))))

;; out-of-bounds? : Ball -> Boolean
;; determines if the ball is completely outside the drawing window
(define (out-of-bounds? b)

  {local [(define ball-left (- (ball-x b) (ball-radius b)))
          (define ball-right (+ (ball-x b) (ball-radius b)))
          (define ball-top (- (ball-y b) (ball-radius b)))
          (define ball-bottom (+ (ball-y b) (ball-radius b)))]
    (or (< ball-right 0) (> ball-left WIDTH)
        (< ball-bottom 0) (> ball-top HEIGHT))})

;; test the program
(start WIDTH HEIGHT)
(move-ball-until-gone
 (make-ball 10 50 50 3 0 'red))
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