Because we use "я пойду" or "я побегу", this kind of thing in the meaning of an immediate action, instead of a planned action, like "ну, я пошёл". This translates into a "shall-phrase", like "Shall I go now?" = Ну, я пошёл уже? or Ну что, пойду я уже.And also, why is Я пойду in your list? That means "I will go" not "I shall" or "I ought to".
This is not quite the same as "I will go" = Я пойду (на концерт завтра)