With WordPress and Make, why are two different interfaces coming up for editing posts

I am designing a blog with WordPress, using the Make theme with a few alterations I have made to its style.css file^*.

My site is not yet online and I have written three example posts only. The following problem has arisen: when I log into wp-admin and start to edit the third post, or when I add a new post (Posts | Add New), I get a different interface from the one I get when I edit either of the first two posts.

Screenshot from "All posts":

As you can see, all three posts are listed. But when I start editing the second, I get this

and when I start editing the third, I get this:

They both seem to be using the Make theme, but they are different. How come? I have done the same thing in both cases: I have hovered over the name of the post ("Second post" or "Third article") and I have then clicked on "Edit". So why doesn't the same thing happen both times? I would like to get the same interface – the one that comes up for the first two posts – when I write or edit any post.

Edit: I think I may be getting the Gutenberg editor for the first two posts and some other editor for the third post and any new posts. I didn't find this explicitly stated anywhere, but the sentence "Access all block and document tools in a single place" seems to be Gutenberg-specific. If someone is reading this who is familiar with Gutenberg, they will probaly be able to confirm from the screenshot. If this is accurate then the question boils down to how can I get the Gutenberg editor for all posts. I have no idea why I am getting two different editors.

^{* Minor things that stop posts from displaying categories, set a maximum width, and set the colour of the dropped cap}