The problem is that when using an editor the content is loaded into an iframe and does not have the context of the surrounding elements and their classes that it would have in the actual page content. So if it gets is styles from classes on the parent elements and those parent elements are not part of the editor content then those styles are not applied in the editor.
So for example if you had a css rule like
.modulecontent p {font-size:20px; }
the problem is that when the content is in the editor it is not surrounded by a div with the class modulecontent but when it is in the actual page it is surrounded by the div with that class and the style is therefore applied in the actual page but not in the editor.
You can compensate for it by adding styles to help make the editor content look the same. The editor iframe has a simple html wrapper around the content but it has a class <body class="wysiwygeditor"
So for example if you want to make use the same large font in the editor as in the css rule above you can add another css selector that will apply the same style. ie:
body.wysiwygeditor p, .modulecontent p { font-size:20px; }
2 selectors separated by a comma using 1 style rule for both selectors.
Hope that helps,
Joe