There seems to be a wide range of customs with regard to reading the haftara. In some shuls (mostly Chassidic) the one who is called up recites the berachah, and everyone then says the haftara quietly to himself. In other places the oleh (or the baal keriah) reads the passage out loud from a single-volume Tanach. And there are yet others who read it out loud from an actual Navi scroll.

Why is there such disparity between these practices? Is there some kind of dispute as to what the haftara reading requires?