get_post_type_object for "post" post_type has no taxonomies

I’m not sure what’s going on here, so I’m looking for advice.

When I implement this code:

$post_type_obj = get_post_type_object('post');
$taxonomy_names = get_object_taxonomies( 'post' );
var_dump($taxonomy_names);
echo "<br><br>";
var_dump($taxonomy_names); exit();

I get the following:

array(0) { } 

array(3) { [0]=> string(8) "category" [1]=> string(8) "post_tag" [2]=> string(11) "post_format" }

This only happens with the post post_type. If I use a custom post type, both dump out identical arrays. Why are the taxonomies missing from get_post_type_object() but not get_object_taxonomies()? Both pull from a global variable, not the database, I believe.

Any ideas/help would be appreciated.

Thanks!