Are you using multiple stylesheets?  

When the editor output doesn't match the DropdownList or the resultant output on the page, we've found it to be that all the stylesheets are not referenced in the 

Editor.EditorWysiwygModeCss

property, or they're not referenced in the same order of the containing page.

It seems the Paragraph and Class dropdownlists resolve the (multiple) CSS references of the *containing* page, whereas the editor will only use the stylesheets references in the above property.

For example, DotNetNuke uses 3 or 4 stylesheets for its final output. All these stylesheets needed to be added to that property for the WYSIWYG editor output to match the dropdownlists.

Hope this helps.

Cheers,