sleep in emacs lisp

script A

    (insert (current-time-string))
    (sleep-for 5)
    (insert (current-time-string))

M-x eval-buffer, two time strings are inserted with 5 secs apart

script B

some comint code (that add hook, and start process)

    (sleep-for 60) ;delay a bit for process to finish
    (insert "ZZZ")

M-x eval-buffer, "ZZZ" is inserted right away, without any time delay

what might have happened? btw, it's Emacs 23.2 on Win XP