I would assume that the preview window renders with the System.Drawing namespaces in .Net. HTML renders via your web browser, obviously. PDF renders via your PDF viewer. So basically, there are always going to be some differences. Part of the report development process is to test the report across a variety of different rendering formats. And I would consider the preview window to be the least important view to get it pixel perfect. Because who looks at it besides the developer?
Perhaps if you describe the exact formatting issue you are having then it will be easier to help.