index.php and page.php in Twenty_Thirteen

Ok I know this topic was commented on many ocations, but I was seeing the Twenty_Thirteen which is by default in wordpress installation and I could see the page.php and index.php looks very similar, I mean they have the same call functions:

page.php

<?php /* The loop */ ?>
			<?php while ( have_posts() ) : the_post(); ?>

				<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>....

index.php

<?php if ( have_posts() ) : ?>

			<?php /* The loop */ ?>
			<?php while ( have_posts() ) : the_post();.. ?>

But I do not understand why? it is not clear

note:I still reading the template-hierarchy but do not explains this.