I already answered to your question in the thread with ID 338600. How ever for your convenience I will paste the answer here as well.
First you need to review the following online help article: Defining custom CSS styles
Now on your problem the reason for this behavior is that you do not have the same style sheets loaded in the RadEditor's content area and in the page in presentation mode. What I suggest you to do is by using developers tool such as IE developers toolbar for Internet Explorer or Firebug for Firefox, check what styles are applied over the problem content then make sure that you have the same cascading in the IFRAME of the editor.
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