Memcached / Memcache Keys Not Expiring

Question

It seems that data expires as expected with Memcached, but the keys themselves never expire. Why are the keys sticking around after the value has nulled? When I run this code:

$frontCache = new Phalcon\Cache\Frontend\Data(array(
    "lifetime" => 30
));

//Create the Cache setting memcached connection options
$cache = new Phalcon\Cache\Backend\Memcache($frontCache, array(
    'host' => 'localhost',
    'port' => 11211,
    'persistent' => false
));

//Cache arbitrary data
$myNewRandomKey = generateNewRandomKey();
$cache->save($myNewRandomKey, array(1, 2, 3, 4, 5));

Taken from http://docs.phalconphp.com/en/latest/api/Phalcon_Cache_Backend_Memcache.html

I can succesfully save the array under the key $myNewRandomKey. I then run this code after the 30 second timeout has expired:

$keys = $cache->queryKeys();
print_r($keys);

And see that the key STILL EXISTS, though the data is NULL, where it was populated before the timeout expired. This seems only half-right to me. The key should have cleared too.

However, If I run $cache->delete($myNewRandomKey); the key will completely erase. Inconsistent!

Bottom line: If the keys don't expire or memcached is buggy, then will they grow and grow over time until it causes a different type of headache - a sysadmin headache?

Stats:
Windows 7 64bit
Apache 2.4.4
PHP 5.4.12
Memcached 1.4.4-14-g9c66c0
Memcache Apache extension version 2.2.7-5.4-VC9-x64 from http://www.anindya.com/category/windows/
Client: Phalcon PHP

Those are the versions, but I'm hoping that's irrelevant to the problem and that my expectations are wrong.

Cheers to all SO'ers!

SL