Often 1 blower housing size will serve different models of furnaces that need to move different amounts of air. Its cheaper having only a few blower housing sizes. Then having 10 different sizes. All they have to do is change the height and width of the blower wheel to change the amount of air the blower moves.ok, fair enough. but it seems to me they could make it so it was more efficient.
perhaps there is a safety reason for it.