Ascending order has strange effects on wordpress loop

It seems the problem is solved by adding 'posts_per_page' => -1 which forces WordPress to show all posts matching the parameters. Here’s my final code:

array ( 
    'post_type'      => 'symposia_program', 
    'days'           => $progDay, 
    'meta_key'       => 'class_number', 
    'orderby'        => 'meta_value_num', 
    'order'          => 'ASC', 
    'posts_per_page' => -1 
);