blog pagination only partiallly works

my ‘next_posts_link’ works for page 2 ok but page 3 gets a 404, why would that happen? thanks

here is my code:

<?php $paged = (get_query_var('paged')) ? get_query_var('paged') : 1; $args= array( 'posts_per_page' => '5', 'paged' => $paged ); query_posts($args); ?>
<!-- Start the Loop. -->
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
<!-- Display the Post's Content -->
<article>
<h1><?php the_title(); ?></h1>
<div class="dashedline"></div>
<?php the_content(); ?>
</article>
<!-- Stop The Loop (but note the "else:" - see next line). -->
<?php endwhile; ?>
<h4><?php next_posts_link('Older Posts') ?> - <?php previous_posts_link('Newer Posts') ?></h4>
<?php else: ?>
<!-- The very first "if" tested to see if there were any Posts to -->
<!-- display.  This "else" part tells what do if there weren't any. -->
<h1>Page not found</h1>
<p>Sorry, that page no longer exists. The page may have been removed or you have followed an obsolete link.
Please use the site's navigation menus to try again.</p>
<!-- REALLY stop The Loop. -->
<?php endif; ?>

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