WordPress | WP_Query does not return anything with s=a

It’s okay here. My request:

SELECT SQL_CALC_FOUND_ROWS  wp_posts.ID FROM wp_posts  WHERE 1=1  AND (((wp_posts.post_title LIKE '%a%') OR (wp_posts.post_content LIKE '%a%')))  AND (wp_posts.post_password = '')  AND wp_posts.post_type IN ('post', 'page', 'attachment') AND (wp_posts.post_status="publish")  ORDER BY wp_posts.post_date DESC LIMIT 0, 10

I tried changing the a to other strings and it worked the same – the query looked for posts where the title or the content contained that string.

I tried it from an external file. My file contents:

define( 'WP_USE_THEMES', false );  
require('wordpress/wp-blog-header.php'); 

$a = new WP_Query( 's=a' );

echo '<pre>';
print_r( $a );
echo '</pre>';