Using setq with global-mode-string

[Decebal]

I have the following code to display the number of lines, words and
chars in the modeline (needs Emacs 22):

    (defvar buffer-count-chars
      nil
      "*Number of chars in the buffer."
      )

    (defvar buffer-count-lines
      nil
      "*Number of lines in the buffer."
      )

    (defvar buffer-count-words
      nil
      "*Number of words in the buffer."
      )

    (defun buffer-count(expression)
      (how-many expression (point-min) (point-max))
      )

    (defun buffer-update-mode-line()
      (setq buffer-count-lines
            (number-to-string
             (+ (buffer-count "\n") 1)
            )

            buffer-count-words
            (number-to-string (buffer-count "\\w+"))

            buffer-count-chars
            (number-to-string (buffer-count ".\\|\n"))
            )
      (force-mode-line-update)
      )

    (unless buffer-count-lines
      (run-with-idle-timer 1 t 'buffer-update-mode-line)
      (buffer-update-mode-line)
      )

    (unless (memq 'buffer-count-lines global-mode-string)
      (add-to-list 'global-mode-string " Lines: "          t)
      (add-to-list 'global-mode-string 'buffer-count-lines t)
      (add-to-list 'global-mode-string " Words: "          t)
      (add-to-list 'global-mode-string 'buffer-count-words t)
      (add-to-list 'global-mode-string " Chars: "          t)
      (add-to-list 'global-mode-string 'buffer-count-chars t)
      (add-to-list 'global-mode-string "  "                t)
    ;  (setq global-mode-string
    ;   (list 'global-mode-string
    ;         " Lines: " 'buffer-count-lines
    ;         " Words: " 'buffer-count-words
    ;         " Chars: " 'buffer-count-chars
    ;         " "
    ;         )
    ;   )
      )

This works okay. But I would prefer to use the commented setq instead
of the seven add-to-list. I would think it does the same, but when
using setq instead of the add-to-list's, only ' Lines: ' is displayed
and nothing else. Why?