Naturally your writing style will change depending on your subject matter and the point of view. However, when we talk about developing your writing style, we mean the voice that is uniquely yours. That voice will change as your writing develops, of course, but like personality, the foundation is already there.
To an editor, on the other hand, style refers to the mechanics of writing, i.e., grammar and punctuation. These rules change depending on what field you're in. For instance, according to Chicago style, used by book publishers, book titles are italicized. Reporters, using Associated Press (AP) style, would put the same title in quotation marks. (Literature students use MLA style, which also italicizes book titles.)
For tips on style, for fiction writers, see: "Developing Your Writing Style."