Why is an article's appearance different in the editor than in Help Center?
The Help Center editor is not a WYSIWYG (what you see is what you get) editor. The editor is, instead, a WYSIWYM (what you see is what you mean) editor. Because of this, the styling of articles, when editing, will generally be different from how it looks when published. The reasoning behind this decision is that content structure and presentation is completely split and that styling options are not done in the editor, they 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.
Therefore the goal of the article editor is not to exactly show how it will look once published, but rather to allow you to build up the content that you then apply styles to afterwards. This ensures consistency across articles, as opposed to using inline styles instead of CSS, which would be difficult to change at a later stage and generally not recommended for web design.
The differences in editor versus published appearance is common when adding tables to Help Center articles.