I can only speculate onto why the labels are "Pages" and not "Posts" but I do know that post is on the list of reserved post types. The fact is that post is already registered by default as your normals Posts so you cannot re-register it. Here's a list of Default Post Types with their respective post type slugs.

What you want to do is actually create your own Custom Post Type. Steps to register Custom Post Type.

I can only speculate onto why the labels are "Pages" and not "Posts" but I do know that post is on the list of reserved post types. The fact is that post is already registered by default as your normals Posts so you cannot re-register it. Here's a list of Default Post Types with their respective post type slugs.

What you want to do is actually create your own Custom Post Type. Steps to register Custom Post Type.