You might be calling wp-cron.php every 15 minutes, but your cron task is set to happen every 5 minutes!
$schedules["5min"] = array(
'interval' => 5*60,
'display' => __('Once every 5 minutes'));
...
wp_schedule_event( time(), '5min', 'isa_add_every_five_minutes' );
Change it instead to every 15 minutes