Right now XWiki displays the view using HTML5 but CK uses XHTML 1.0. This can lead to differences in the display since HTML5 elements are ignore in XHTML 1.0. See attached screenshots for the following example:
(I applied some custom CSS to show the difference - Note that it's possible that CK doesn't use the page's CSS so the example might be moot on the visual aspect difference in this case but the issue still stands - In this case the browser simply ignores the figure and figcaption tags).
See also https://dev.ckeditor.com/ticket/9457