There are a number of peculiarities in the UI that are intrinsic to the specific execution platform. A Windows application and an OS X/macOS application may have the same functionalities but have a different user interface. They wouldn't necessarily have a completely different UI; just keep in mind that every platform has its own conventions and look and feel particularities that you need to respect if you want to provide a UI the user feels is natural in the overall picture of the operating system (OS) they are acquainted with. This will greatly impact the user-friendliness of the application.
FireMonkey's promise is to let you build a UI application that is cross-platform, meaning the same application will be deployed on several platforms without having the very same aspect on each of them. The application will be transposed into the specific look and feel of each platform without requiring a complete rewrite of the application...