WordPress creating images if uploaded image is greater than 960px on one side?

Question

I'm creating a new theme and have modified the media settings. From the Settings > Media page, all sizes are marked as 0x0 so they will not be produced.

I am creating only 1 custom thumbnail in my functions.php file. In total I should be expecting only 2 images uploaded to Wordpress (the original file and the post-page-thumb)

After uploading an image through the media library (test1.jpg), I see that there is an additional image being created by Wordpress 768x960 when I go in FileZilla.

[![filezilla][3]][3]

My functions.php also includes the ability to view custom images when adding media to posts/pages (see above), however, this additional size is not available from the drop down (only the original and thumbnail created).

[![add media][4]][4]

My assumption was that WordPress internally does this somewhere for images that are as large as the one I upload (1600x2000). I don't have any plugins activated.

I tested two additional images: test2.jpg (at 768x960) and test3.jpg (at 600x750). test2.jpg still created the additional 768x960 image, while test3.jpg didnt.

[![new files][5]][5]

Does anyone know what can be causing this issue or a way to fix? Is this an issue with WordPress 4.6.1? I've never experienced it before.

New user so can't include more than two links, but all are viewable here:
https://i.sstatic.net/SQOo6.png [1]
https://i.sstatic.net/05ZRm.png [2]
https://i.sstatic.net/TiOLA.png [3]
https://i.sstatic.net/HJwSU.png [4]
https://i.sstatic.net/bZl69.png [5]

Answer 1

WordPress v4.4 introduced an additional image size named medium_large which is is 768px wide by default, with no height limit. There is no UI for the medium_large but the size can be changed by updating the options:

/**
 * Updates the medium_large image size. 
 * It's only necessary to execute this code once.
 * Ideally you'd want some kind of UI to allow the dimensions to be specified by the user.
 */
function wpse238808_update_image_size_medium_large() {
    update_option( 'medium_large_size_w', 0 );
    update_option( 'medium_large_size_h', 0 );
}
add_action( 'init', 'wpse238808_update_image_size_medium_large' );

Changing medium_large's width and height to 0 like you've done with the other image sizes will prevent the additional images from being generated.