WordPress MU

I set wp_schedule_event to go off at ten minute intervals. It records the time at which it went off. I am noticing that event occurs multiple times between the ten minute intervals. I checked all of the cron processes and I am sure that there is no duplication. Why is the job happening too many times?

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() {
	update_site_option('zzz-every_ten_minutes', date('m-d-y H:i:s A'));
}