Why content_arr[‘extended’] removes paragraph tags?

seems when i add the following using wpautop to the output it resolves the problem. i would love to know from someone smarter than myself why this fixes it. why when we just use the_content the p tags are included, but when we use get_extended do the p tags get removed?

$content_arr = get_extended ( $post->post_content );

            if( strpos( get_the_content(), '<span id="more-' ) == true ) {
                echo wpautop($content_arr['main']);
                echo '<div class="morecontent">'. wpautop($content_arr['extended']).'</div>';
            }
            else {
                the_content();
            }