The editor is using the page styles rather than the defined styles in the stylesheet that is referenced in the editorwysiwygmodecss property. Specifically... If the stylesheet is in the SAME FOLDER as the editor page, it works (the stylesheet styles overload the page styles). If not in the same folder, the styles in the stylesheet have no effect. However, the stylesheet file must be there, or you get an asp.net error, but the editor does not apply the file style definitions to the editing area.
For example,
Editor1.EditorWysiwygModeCss = "~/temp/stylesheet1.css" does not work
Editor1.EditorWysiwygModeCss = stylesheet1.css" does work
Seems like a simple bug to fix.