Historically doors measured true to their description and size. Wooden doors are still made to true size, so if you are looking for a 36" x 80" x 1 3/4" wood is what you are looking for.
Fiberglass Doors are shy of 1/4" of a true size, for example a 3/0 would normally measure 36", but NOT for a fiberglass this would be 35 3/4" and is only 79" in height. This difference of a quarter inch in width and 1" in height can cause headaches and problems for a lot of people, especially if they just want to make an easy door switch from an old broken door to a newer one. When a person is in need of a new door and wants to simply change out their existing door for a fiberglass door, they may run into a sizing issue.
I don't have any good reason why the sizes aren't true, but I do know that Dayna's Doors can add to both the bottom and edge of the door to make it a true size. Dayna's Doors is able to adjust and help make those adjustments, because the fiberglass exterior doors are such a great product making the change is well worth it!
