Differences
The big difference here is User Interface vs. under the hood programing. Basically when a programmer is creating a program to compete against a Microsoft or Apple counterpart, it is certainly wise to use a similar ui design, given that people like to use what they are used to, and it does not hurt to use Redmond Experience in user interface, we have to admit they don't do it bad. But under the hood, that's another story, if you're developing a clone of a windows application then you try to give a better experience in the subtle details, like making it work faster, prettier icons, or being less a resource hog. That said, if you compare a linux desktop with a windows one, linux needs less resources and make a better use of ram, but certain applications as openoffice fall far behind the performance of their windows counterpart.
Bottom line: you copy windows ui, but the chance to be a better outlook than outlook is inside your app, and that's where evolution is better.
