The UI's are different between the OS X version of Dreamweaver and the Windows version because of the different document interfaces that the operating systems use.
When you're on Windows, Dreamweaver uses a Multiple Document Interface... it lumps everything you're doing into one big window. On OS X, Dreamweaver uses a Single Document Interface, because that is the way things have been on OS X, and IMO, is much better than the MDI. This is how, for example, when you're in one application and can "see through" into another app. Like if you're in Photoshop working on a design, you can see behind it, which you can't do on Windows, because it places everything into one "Parent" window.
Understand?