random post ceases to be random at subsequent refresh

<?php
$args = array( 'posts_per_page' => 1, 'orderby' => 'rand' );
$rand_posts = get_posts( $args );
foreach ( $rand_posts as $post ) :
setup_postdata( $post ); ?>
<?php the_content(); ?>

Wrote this a couple of months ago, in line with some advice I got from documentation&forums, with the intention to display the content of a random post – and it worked.
The thing is though I am now noticing that the “randomness” only applies to the first time I visit my site – not at subsequent refresh.
That is, no matter how many times I refresh my site, I get the same post, that was originally loaded as random.
Obviously, my question is: Why is this happening?
I’m thankful for all the help I can get.