Why is the shutter for my Air Vent whole-house fan a different size from the whole-house fan itself?

The rough opening that is cut for the shutter is a different size so the whole-house fan can be pushed through the opening from inside the hallway rather than try to get it through a smaller attic opening.

The rough opening that is cut for the shutter is a different size so the whole-house fan can be pushed through the opening from inside the hallway rather than try to get it through a smaller attic opening.