Trying to figure out why WP is using unregistered file sizes

Hello, before running this plugin, I took a look at what media sizes my site is using and what are the sizes it is not using.

I only have three sizes registered in SETTINGS > MEDIA

But some of my pages are using sizes that are not in the above list. For example, the cover block in my homepage is using 768W and 800W and 1536W.

So I’m confused why these other sizes are there (perhaps from older plugins and themes i used in the past?) and why does WP is using them.

It seems to me, first I need to figure out how to tell WP to use the registered sizes before I run your plugin. Any advice you might have, I’d really appreciate it.

The page I need help with: [log in to see the link]