Why is an article's appearance different in the editor than in Help Center?
The Help Center editor is not a "what you see is what you get" editor. Instead, the editor is a "what you see is what you mean". The styling of articles while editing is generally different from how it looks when published.
The content structure and presentation are split. All styling options are done in the Help Center CSS which is then applied to the content upon publishing. This is the same principle that you find when working with Markdown: the intent versus the finished product.
The goal of the article editor is not to show how the article looks when published, but rather to allow you to build up the content that you then apply styles to afterward. This ensures consistency across articles, as opposed to using inline styles, which are difficult to change at a later stage and generally not recommended for web design.
The differences in editor versus published appearance are common when adding tables to Help Center articles.