WordPress post editor looks different than live preview

By “preview” I’m supposing you’re referring to what you see in the editor itself, and not what you see when you click the PREVIEW button at the upper-right corner of your screen.

If so, then what you’re seeing is normal behaviour.

The “look” of your site is ultimately defined by your THEME, not WordPress itself.

The old editor (aka Classic Editor) was just a box (like a word processor) to type in your post, and there was no way to see how the post would look like on your site until you published it (or clicked the PREVIEW button).

The new editor (aka Gutenberg) is trying to solve this “problem”, and the ultimate goal is the sort of parity you’re looking for (as well as being able to design the entire page layout).

But even then, this parity will not come automatically. The theme’s author will need to add the necessary code to their theme and take care to overwrite any default styles that any block may have — to ensure that what’s in the editor will match what’s on the live site.

Yeah, so it’s ultimately up to your theme; and some themes (eg the default “TwentyTwenty One” theme) are way ahead on that “parity” benchmark than others.

I personally wouldn’t sweat over this.

If you really must see how your post/page would look like BEFORE publishing, click the PREVIEW button in the editor. This will open the yet-to-be-published post on your site for you to see.

Good luck!