Depth of Windows jamb

[jimmyscottage]

Most people have 4" wall (i.e. 3.5" stud). Therefore most exterior door in home depot is about 4.5" (4-9/16 to be specific), which include 3.5" stud, 0.5" drywall, and 0.5" sheathing. This is pretty easy to understand. However, for exterior windows, they have all kinds of jamb depth. Some of them is 3-1/4", some of them is 3", lots of them over 6" (not jamb extension), some RSO (Complete-Tear-Out Window Installation) window does not include jamb at all (about 1.5" out side of nail fin, about 1.5" inside of nail fin, so that I need to build the window frame for those RSO window). Why is it so complicated?