wp_schedule_event happens multiple times (wp cron)

[mdgross]

wp_schedule_event is occurring frequently between scheduled intervals. I have it set to record its execution time every ten minutes.

In a plugin file:

add_filter('cron_schedules', 'more_reccurences');
add_action( 'happens_every_ten_minutes', 'do_this_every_ten_minutes' );

function more_reccurences() {
	return array('tenmin' => array('interval' => 600, 'display' => 'Every Ten Minutes'));
}

if (!wp_next_scheduled('happens_every_ten_minutes')) {
	wp_schedule_event( time(), 'tenmin', 'happens_every_ten_minutes' );
}

function do_this_every_ten_minutes() {
	$times = get_site_option('zzz-every_ten_minutes', array());
	$times[] = date('g:i:s A');
	update_site_option('zzz-every_ten_minutes', $times);
}

On a template page:

foreach(get_site_option('zzz-every_ten_minutes') as $execution_time) {
	echo $execution_time . '<br />';
}

The result of recording execution time at ten minute intervals is:

8:34:56 PM
8:34:59 PM
8:35:04 PM
8:35:12 PM
8:35:17 PM
8:35:28 PM
8:35:33 PM
8:35:39 PM
8:35:47 PM
8:36:06 PM
8:36:07 PM
8:36:27 PM
8:36:48 PM

As you can see the event is happening much more frequently than every ten minutes... what is going on here?