'priority' doesn't work

Below is my configration in global.php

return array(
'service_manager' => array(
'abstract_factories' => array(
'Zend\Log\LoggerAbstractServiceFactory'
)
),
'log' => array(
'Application\Web' => array(
'writers' => array(
array(
'name' => 'stream',
'priority' => 5,
'options' => array(
'stream' => 'log/app.log'
)
)
)
)
)
);

I tried to set priority from 1-1000, it'll still output message to log file in all level.

What am I doing wrong? testing code below:

class IndexController extends AbstractActionController
{

    public function indexAction()
    {
        try {
            $logger = $this->getServiceLocator()->get('Application\\Web');
            $logger->emerg('test');
            $logger->info('aaa');
        } catch (\Exception $e) {
            do {
                var_dump($e);
            } while ($e = $e->getPrevious());
        }
        return new ViewModel();
    }
}

if I set the 'priority' => 1

in app.log:

2014-03-29T19:31:54+08:00 EMERG (0): test
2014-03-29T19:31:54+08:00 INFO (6): aaa

if I set the 'priority' => 7

in app.log:

2014-03-29T19:32:57+08:00 EMERG (0): test
2014-03-29T19:32:57+08:00 INFO (6): aaa