[Plugin: WP-Paginate] Different next post / previous posts in index.php and single.php

When I replace the “next post / previous posts” on my theme’s index.php

<?php next_posts_link('&laquo; Older Entries') ?> | <?php previous_posts_link('Newer Entries &raquo;') ?>

with

<?php if(function_exists('wp_paginate')) {
    wp_paginate();
} ?>

Pagination works GREAT!

When I replace the “next post / previous posts” on my theme’s single.php

<?php previous_post_link('&laquo; %link') ?> | <?php next_post_link('%link &raquo;') ?>

with

<?php if(function_exists('wp_paginate')) {
    wp_paginate();
} ?>

NO Pagination shows up.

Is there a difference between the two previous_post_link & next_post_link in the index.php, archive.php, single.php etc.

http://ww.wp.xz.cn/extend/plugins/wp-paginate/