Understanding dotimes skipping by 2

[Tim Johnson]

using GNU Emacs 26.1 (GTK+ Version) on ubuntu 14.04
 
the following code snippet is as follows:
(setq l `(1 2 3 4 5 6 7 8 9 0))
(1 2 3 4 5 6 7 8 9 0)
;; iterate through a list two elements at a time
(let ((x 0))
  (dotimes (/ (length l) 2)
    (progn
      (insert (format "%s %s, " (nth x l) (nth (+ x 1) l)))
      (setq x (+ x 2)))))

;; and below are the results
1 2, 3 4, 5 6, 7 8, 9 0, nil nil, nil nil, nil nil, nil nil, nil nil, 2

I'm confused about the output (nil etc...)which follow the expected numbers. 
could someone explain?
P.S. I get the same output without the `progn form
thanks
-- 
Tim Johnson
http://www.tj49.com