It is for residential use. The doors were made to order, from 2 different manufactures. Both said the door had to be 6'7".You can get a fire door nearly any size that you want. That being said, I assume that you bought an "off the shelf" door and it is sized for a residential type threshold.
I don't understand it either. All the doors we're ordered, new pre hung. The doors were all wood. Every door is 6'8" except the fire door. We keep all of our door heads at the same height which presents a problem when the fire door is an inch shorter. The options are to cut all the doors to 6'7" in that area, hang the fire door lower and make the head casing larger or add a piece to the bottom of the fire door. Why? Why can't the fire door just be 6'8' tall?great question that I've wondered for years. All steel and fiberglass doors are 79" while wood doors are 80". My guess is that it has to do with the threshold. Steel doors can't be easily cut so they fit into a frame and a threshold prehung while wood doors used to be installed onsite knock down and threshold and copper weatherstripping custom fit on site and the door trimed to fit. Never quite figured it out myself.