Editor preview and page or post actual look different

I run linux as my main operating system and edit my website from it. I have always wondered why when working on a page or post it never looks like the actual page or post when you publish it.

Things I have noticed : Pages and posts that you are editing it seems only half the available space for the body of the page/post is usable. Then it wants to wrap it all to the next line. But when you publish it it will sometimes all be on one line. Perhaps I am missing some configuration or something. Seems I cannot use the full width of the editor for pages and posts??

If it matters, I am using the Business pro template and is located at https://www.renuecomputers.com

The website is still under construction.