You are complaining that the drop-down menu of grades does not contain enough choices. Other people complain that it contains too many choices. We can't win!
If you want to change the list of choices in that menu, then edit the ensure_fraction_options_initialised method in question/engine/bank.php (around line 333) in Moodle 2.1. In earlier versions, edit the get_grade_options function in lib/questionlib.php (around line 410).