This is being caused by default css that TinyMCE is applying to your elements within the editor itself.
You can supply your own css file though as a configuration option when you initialise the editor using the content_css option: http://www.tinymce.com/wiki.php/Configuration:content_css
I don't know the default css styling that TinyMCE is using within your editor. But if you create your own content_css file you should be able to use that to set css that would match the defaults you would normally see in a browser.