Simple question about proper $conf variable setting.

Through some trial and error, I learned this works:

$conf = array(
'cache_inc' => 'sites/default/modules/memcache/memcache.inc',
  'memcache_servers' => array(
    'localhost:11211' => 'default',
  ),  
  'memcache_bins' => array(
    'cache' => 'default',
    'cache_filter' => 'filter',
    'cache_menu' => 'menu',
  ),  
);

But this does not:

$conf = array(
'cache_inc' => 'sites/default/modules/memcache/memcache.inc',
  'memcache_servers' => array(
    'localhost:11211' => 'default',
  ),  
  'memcache_bins' => array(
    'cache' => 'default',
    'cache_filter' => 'default',
    'cache_menu' => 'default',
  ),  
);

I'm not sure why the former isn't the one that is broken, though. Don't you assign a bin to a cluster? "Filter" and "menu" are not clusters.