Getting posts based upon category returning wrong content

I’m writing a widget which will get posts based upon the category they are in but I am getting results back which belong to different categories.

In order to get the categories I’m using

foreach( $slugs as $slug )
{
	if( $slug != '')
	{
		$category = get_category_by_slug( $slug );
		if( $category !== false )
		{
			$category_list[] = $category->cat_ID;
		}
	}
}

This give me $category_list containing the following data:
array(2) { [0]=> string(2) “32” [1]=> string(2) “37” }

I am then using

$related_posts = get_posts( array(
				'numberposts'  => $total_to_return,
				'category__in' => $category_list
));

which is returning me $num_to_return posts regardless of which categories they belong to.

I’ve also attempted to use

$post_args = array(
		'posts_per_page' => $total_to_return,
		'category__in'   => $category_list,
		'orderby'        => 'post_date',
		'order'          => 'DESC'
);
$get_posts = new WP_Query;
$related_posts = $get_posts->query( $post_args );

which returns the same posts. I’ve poked through wp-includes/query.php and wp-includes/post.php to try to figure out what is going on without any luck.