There is a big difference in UI design for the mobile devices and for the desktop. On the desktop you can often specify the UI using automatic layout features and the result works. On mobile devices the limited screen estate forces you to weight every pixel and therefore the different screen sizes matter much more. Similar argument can be made about the performance – on desktop Macs the performance differences are not interesting for most regular apps, whereas on the mobile devices even a simple app can tax the machine to the point of not being usable.