Apple has always been the mobile provider that has not been associated with "fragmentation". You see other mobile OS's out their (Android) with multiple screen sizes and customized operating systems, with multiple issues that can cause major headaches for developers. Well, it's all about to change.... With the recent launches of iPhone 5, iPad Mini, and iOS 6 we see major changes in development that have to accommodate and factor in more development time.
It is inevitable though, Apple simply can't have one screen forever. As the marketplace becomes more and more saturated by smart phones the iOS series has to adapt to the ever-changing marketplace.