The common way of installing (interior) doors in Africa on a construction site is to put the frame up before laying the bricks. The frame is used to determine where the interior walls should start and how wide the door opening should be. Come rain, wind and sunshine, come wheelbarrow, bricks and concrete: the frame is there, waiting for construction to finish so the actual door can be hung. When that time finally comes, a carpenter arrives to hang the door. The patiently waiting frame (be it wood or steel) is no longer meeting the size of the pre-cut door slab, because of all the weather it faced. So the carpenter sands, planes and chisels, while testing every now and then if the door fits the opening. Finally, when the door is on the hinges he can determine where the openings for the lock and handles should come. You can imagine: no 2 doors look the same after this.
Now with a pre-hung door this is different. A pre-hung door needs no work on site, other than hanging it on its hinges and installing the lock and handles in their pre-cut openings. To be able to use a pre-hung door you need to use a different kind of frame, called an architrave frame.